Allowing them on the platform doesn't mean a full endorsement of the belief. It means that he (or whoever makes the decision) finds the belief acceptable enough to platform.

There is likely some line which is too far, and not allowed on the platform. Perhaps "eating live babies"? "Kicking puppies"? Something that is so unacceptable, it would not be allowed. This argument is that ICE and Nazi stuff belongs on the far side. That as a platform owner, you can say "that's not allowed here".

Allowing one person to say "I think the NY Yankees are the best" and another to say "I think the NY mets are the best" on your platform (eg: website, newspaper, bulletin board) doesn't mean that you personally believe both. But if you let someone post "I think white people are best" and just leave that up, that's saying that's an acceptable message to say. Just harmless like talking about baseball.

This argument is some positions, like what ICE is doing, is outside the range of acceptable. The platform (a website in this case) should say they have to take that elsewhere.