Fiduciary duty is a responsibility to act in someone's best interests (the investors in this case)[1]. The requirement to not make misleading statements affecting investors is a different requirement [2].

To the substance of your statement, they don't shadow ban, so that's not a misleading statement. The fact that a lot of people take an extremely broad view of what shadow banning is doesn't change that. Twitter actually define shadow banning in the link in the tweet you posted, and explain what they do and why that's not shadow banning.[3]

[1] Very good explanation here https://www.investopedia.com/ask/answers/042915/what-are-som...

[2] Not to "misrepresent" material facts, which is a form of securities fraud https://www.investopedia.com/terms/m/misrepresentation.asp

[3] The original link is down but I found it in the internet archive. https://archive.vn/m4bT4