You can read the case. Basically it was a privately owned public space that they could have been otherwise trespassed from, but not for the reason of their speech. Since the reason for the trespass was their speech, it was prohibited. They were not otherwise "forbidden" from walking there were it not they expressed something that was disapproved of.

A weak analogy (I know analogy are never allowed here because "they're not the same") is that you can fire someone at will. Unless it turns out you fired them because they are black (yes I know being black is much different than expressing an opinion). It didn't mean you can't fire them at will, just that you couldn't for that specific protected reason.

Although at this point we're well well past the goalpost of "Freedom of speech has literally never prevented a private company from controlling the content on its platform" and down into the weeds of how it happened. The case clearly prevented the company from fully controlling the content of its sidewalk platform.