To perhaps put this another way.

Would you permit someone to publish on Substack who advocated lowering the age of consent to, say, 8? Who would then describe, in detail, the specific acts this would permit him to perform?

That would certainly count as “free speech”. Would you let him publish?

Or are there areas of speech that you consider beyond what you would permit Substack to be associated with?

Substack is a community. As the owner of the land where this community resides, you have both the right and the responsibility to decide who is to be admitted, and who is not.

By setting standards you are not “making the decision” what I or anyone else reads. You do not gatekeep all publications - it is sheer arrogance for you to imply that you do. Those who wish to read what you choose not to publish are free to go elsewhere.

You are empowered to show the Nazis the door. That you have this power and choose instead to admit them is an unwise choice.