The major difference is that in the type of reading Joel Splosky is talking about, you are coming in not knowing the code's intent. It was written by one or more other people at some point in the past, likely with many iterative changes over a period of time. Figuring out the intent in this case is 90%+ of the work. With LLM generated code, you know the intent. You just told the assistant exactly what your intent was. It's much, much easier to read code that you already know the intent of.