When someone says someone is legally trespassing read it as “legally [speaking they are] trespassing”. At least in most cases.

Pedantic tangent:

You could lawfully trespass on the land of another (with permission). There’s 4 elements to the tort of trespass to land. 1) You act volitionally. 2) You intend to occupy that space, are substantially certain that will happen as a result of your actions, or you intend another intentional tort granting transfered intent. 3) But for your act their property wouldn’t have been invaded. 4) Their property has been invaded.

In civil law a trespass to land doesn’t consider whether you have permission or not to determine if you trespassed. They would determine that you did infact trespass but you have the defense of having done so with the privilege to do so granted by the owner. Meaning you did trespass but did so only in a manner appropriate under law.