The point is valid about performance over time changing, but using an LLM to see if a number is prime is a terrible use of the product. It does, of course, give you a correct answer if you have the wolfram alpha plugin installed, though.
Solving an IQ test is also not useful by itself, but it is a good benchmark of intelligence.
This is not well-established, and is subject to a great deal of dispute amongst experts in the field.