True. If nobody else is told about it it's not Libel.

So they would have to stop that person not in public or very very discretely, only one person could be involved from their side (otherwise somebody would be telling somebody else) and the person stopped could simply refuse and walk away because if they called the authorities by telling them that person had stolen something, it would be Libel (or worse - I believe it's also a crime to falsely report a crime).

Ultimately, it boils down to had they witnessed it or not - had they seen it on camera then they did, had they just blindly relied on the AI flagging it without actually checking it, then they did not, and "the AI told us so" wouldn't fly in Court if they didn't cross-checked the AI system and it turned out they had made an accusation of a Crime against somebody to somebody else or the authorities without actually checking it.