What guarantees it’s intact?

It doesn't need to be. If the license isn't positively exactly permissive then you can't use it.

Can you even trust that the License in a random repo is accurate and expresses the actual copyright of all the contained code?

I guess my point is, you can't be positive that even if you're following the license in a repo you forked that the repo owner hasn't already violated someone else's license, and now transitively, so have you.

> Can you even trust that the License in a random repo is accurate and expresses the actual copyright of all the contained code?

In fact, that seems to be exactly the problem shown in the tweet - someone copy-pasted the quake source and slapped a different license on it, and copilot blindly trusted the new license.