Exactly, they aren't good at creating new material. But many discussions in comment section are simply regurgitations of existing material, which they are good at rearranging. New novel discussions in places like this are actually a very rare thing, as many comment sections are simply people who already know informing those who don't. I'm doing that right now, funnily enough.
No, they aren't even good at rearranging existing material. They produce bad writing that only superficially looks good in a lowest-common-denominator sense, and falls apart under any close examination. Everything is wrong with it, from the sentence structure to the rhetorical forms to the substance. AI 'writing' is a loose collection of cheap tricks that score well on A/B.