If you actually read the release notes, you wouldn't say this.
1. You don't seem to think the Pale Moon has value, nor that it should be used.
2. You say you don't read their release notes--which makes sense if you don't think it's worthy.
However:
3. You say you do spend time reading their patches--which doesn't make sense if you don't think it's worthy.
4. Their release notes frequently mention "defense-in-depth" patches which are their own work, not, as you put it, to "randomly backport patches purely to try to keep somewhat up-to-date"--which conflicts with your claims about their work.
So you say you read their patches, yet you don't appear to read all of them. And you say you don't read their release notes, yet you speak as if you have comprehensive knowledge of their work.
Then you say something that's supposed to be scary, but then you say that it might not actually be scary, and you won't tell us whether it actually is.
So, regardless of whether Pale Moon is valuable or useful or secure to any degree, isn't your comment a textbook example of FUD? What's your purpose here?
If I really wanted to scare you, I'd tell you about such security-friendly-sounding commits as "we don't want the newest release of the [crypto] library, let's freeze at an older version" or "re-enable support for RC4 in TLS." But that would be unfair, because you'd actually have to read the commit to judge for yourself if I'm quoting them in good context.