This completely ignores the influence that flagship instances have.

People don't become Neo-Nazis without spending time among other Neo-Nazis listening to Neo-Nazi talking points. People don't become Tankies without spending time among other Tankies listening to Tankie talking points.

People are more likely than not to join the flagship instance than any of the smaller instances and significantly more likely than starting their own instance. Some of them will realize the kind of people they have found themselves surrounded by and leave. Many more won't - especially due to general ignorance about Tankies (see this entire thread and the absolutely bizarre common defense of Tankies not being equatable to Nazis when they absolutely are and have resulted in more deaths and suffering than the Nazis ever have or will.) The people who stick around might start thinking the Tankies make some good points. This is how you end up with more Tankies. More Tankies is always a bad thing just like having more Neo-Nazis is always a bad thing.

Most people have a good sense to not surround themselves with Neo-Nazis. That good sense appears to be missing when dealing with Tankies.

Apart from everything else that has been mentioned I would add that a very real factor is that every instance to my knowledge has a donation button which cannot be removed without forking the project that goes directly to the devs. Plenty of people in this post are saying things like "Wow, glad they cleared that up! I'll definitely be giving them money" and very likely don't have the full context. Assuming Lemmy does continue to grow exponentially it wouldn't be unreasonable to assume that these devs will probably receive a large amount of money in donations from people who may not all be okay with supporting them if they were more informed. So spreading the truth about the devs and not taking their pitiful denial as absolute truth is important even if you believe in the project as a whole.