To their credit, I think the Principles of Communism thing is partially meant as a floodgate, since the devs really do believe in their project and want to avoid over-centralization from everyone defaulting to one instance. They know many people will go "What the hell? No!" and go somewhere else and that's exactly the point. I'd be surprised if they really thought it would get almost anyone to engage with Marxism with the prompt, especially since you can copy the first sentence of the text and not read anything else (and even just reading it is not engaging with it). I think it's more like a little joke.
Also, copying a sentence of your choice to a pamphlet is not a pledge and I think it's silly to view it that way. If it helps, iirc, one of the sentences that appears is "No." and they will accept that as an answer.
But assuming this was "promoting an ideology directly," would you find it less sketchy for an instance to promote ideology indirectly? Because if you aren't directly doing ideology, that just means you are indirectly doing it (sometimes very deliberately). Personally, I appreciate transparency.