My short response to this is that you can let people speak freely while not trusting them to run platforms where there are clear conflicts of interest.
I do not care that tankies exist. They can have their communities, they can run their communities, they can have their platform and try to make their point. That is fine, that is good, that is healthy.
They should not, however, be running the platform that you are trying to speak on. Lemmy is a platform that you are going to be trying to speak on, and it is a platform that they have control and power over.
As people who support authoritarian control, that power is going to lead to the sort of authoritarian monoculture that you do not want to see happen.
So you support the platforms that are actually pro free speech, that are actually not having this massive conflict of interest. You support platforms which you can trust to actually care about the values they are promoting.
Lemmy is not such a platform, you should not support it. Because if you do, they will close that door and they will empower the sort of authoritarianism that you fear will occur.
Adamantly defend their right to exist and speak freely, adamantly defend your own right to ensure they never hold any form of power, because they will take it and they will destroy you with it.