Imagine you are running a forum.
A few users decide to use it to exchange some encrypted content,impenetrable walls of base64.
Should you be able to ban them, and remove their postings, if they don't listen to polite requests to stop?
How is this different from being an email service, or an ISP?
I mean, there are more relevant details than the very broad formulation assumes.
However, as a forum owner, I have the Right to Filter the content and users on my forum, so I can ban them. And they're free to go to another forum and communicate that way.
Now, let's say that my forum is ridiculously popular, and completely entrenched, and that I take active steps to kill or buy any competing forums, and eventually my forum becomes the only way to share content online. I still have the Right to Filter, but you could argue (and I personally would argue) that my existence as a monopoly is incompatible with the Right to Communicate. Essentially, I have privatized a public resource, and my forum should either be broken up or (if that's not possible) regulated like a publicly owned resource rather than as a private one.
That last point is where ISPs become interesting. ISPs aren't nationwide monopolies, but they are often regional monopolies. If your only choice of provider is Comcast, and Comcast doesn't follow Net Neutrality, then I would argue that Comcast's existence is incompatible with the Right to Communicate. You don't have a choice of going somewhere else -- if Comcast filters, you just flat-out can't access public content.
It is very, very difficult to break apart ISP monopolies. Additionally, ISP monopolies consume large amounts of public funding and subsidies, which means the public can make a strong claim that they are borderline essential, public utilities, at least partially funded through public taxes and city-wide contracts. And without a neutral ISP that everyone has access to, the Internet just flat-out doesn't work. Because of that, I personally support government restrictions that force ISPs to respect my Right to Communicate. But if someone thought that was a bridge too far, I wouldn't say they were stupid. The ambiguity on that issue in the manifesto is on purpose; I want guiding principles rather than a step-by-step tutorial on every problem.
In regards to email, the same principles apply. Is the current system making it impossible for users to (voluntarily) communicate with each other? Spam doesn't count here, the Right to Communicate is not a right to force everyone else to listen to you. What is concerning about email to me are cases where untrusted IPs mean that multiple parties who want to communicate with each other are getting emails bounced and/or silently rejected with no way to fix the problem or bypass it.
If you wanted to make a case for regulating an email service with some version of Net Neutrality, you would need to make a case that email is a fundamental public utility that users/communities can't just avoid or bypass, that providers like Gmail/Yahoo are so big that they can't be broken up or unseated by normal means, and that they're currently failing to meet the standards that their size/power demands. I think making that argument about email providers is a lot tougher than making it about an ISP. But again, different people might reach different conclusions.
My short answer is that where public communication (one-to-many) is concerned, the Right to Communicate:
A) describes voluntary communication. It doesn't give you the right to force someone to listen to you, it gives you the right to share information with someone who wants it. (I would be open to modifying the language if that distinction isn't currently clear.)
B) are designed to combat legal challenges (Facebook may ban you for talking about something they don't like on their platform, they shouldn't be able to sue you).
C) are designed to draw attention to infrastructure and monopoly problems (the existence of one company that controls a large majority of web content is in itself a threat to these rights).
These rights are purposely vague because I want them to be stronger than just my own opinion; I want them to be somewhat interpretable and adaptable.
However, in my opinion if I want to publish something and I have to publish it on Mastadon instead of Twitter, I don't consider that to be a violation of my Right to Communicate. If Twitter becomes the only way to publish content, or it controls so much of the Internet that publishing outside of Twitter is infeasible, then in my opinion it's reasonable to start talking about antitrust.
There's leeway there -- ISPs are always going to be gatekeepers just because of how the Internet is structured, so maybe we decide that it's OK to force Net Neutrality on them. In general though, we should break apart monopolies rather than force them to be neutral.
Bear in mind that the Right to Communicate is also about the right to encrypt message contents and to conceal message recipients, and to circumvent platform restrictions that try to remove those rights. The right to publicly publish information is only part of that, and is (imo) an already relatively well established right that I don't lose much sleep over. Other people may disagree with that, but I don't think Twitter/Facebook have as much control over modern communication as is often supposed.