The question is who comes up with words. If you re-type textbook, you are plagiarizing. Same happens if you re-type ChatGPT output.
On the other hand, if you read some text first (be it ChatGPT's output, or a textbook) and then rephrase it yourself, then you are the author.
How much you have to rephrase? Is changing every other word with synonym enough? That's actually a gray area, and it depends on the teacher. Most teachers would expect you to at least change sentence structure. But in this case it's completely irrelevant, as we know the students did copy/paste.
I really don't see why you are trying to present ChatGPT like something special re plagiarism. Copying other's work is copying. Paying $10 to someone to do your homework and then copying their answer as-is is cheating. So is using ChatGPT yo do it for free.
ChatGPT is not someone. It's a tool.
So is a textbook. Still not OK to copy homework from it.
Textbook has an author that you can copy. You can't copy output of an auto suggest, it's just yours.
It does not matter if author is human or computer.
If there is a spelling bee, but student is secretly using spellcheck on the phone, they are cheating.
If there is a math speed competition, but student is using a calculator on the phone, they are cheating.
If it's a calculus exam, but student is using Wolfram Alpha (or TI-89) to calculate integrals and derivatives, it is cheating.
If it's a written exam but student is using ChatGPT to write the text, it is cheating as well. Not that different from previous cases.