That is kind of a no brainer rule. Otherwise you’re going to see a lot more late hits and quarterbacks getting injured when defenses see them try to slide but aren’t sure if they really are and go ahead and hit them anyways.
I remember Zach Wilson did that a few years back. Julian Blackmon hit him on his next slide on the same play. Zoobs were so mad that he didn’t get flagged. The Wilson’s credit, he wasn’t butt hurt about it because he figured he faked it the first time so he got what was coming to him.