I seriously doubt that they think senior reviewers will meticulously hunt down and fix all the AI bugs. Even if they could, they surely don't have the time. But it offers other benefits here:
1. They can assess whether the use of AI is appropriate without looking in detail. E.g. if the AI changed 1000 lines of code to fix a minor bug, or changed code that is essential for security.
2. To discourage AI use, because of the added friction.
Point 1 is important. Seniors (or any developer really) with experience in the code base in question can judge pretty quickly if a CR seems reasonable.
1. They can assess whether the use of AI is appropriate without looking in detail. E.g. if the AI changed 1000 lines of code to fix a minor bug, or changed code that is essential for security.
2. To discourage AI use, because of the added friction.