That's a motte-and-bailey defense. Yes, what you say is
technically correct with respect to meaning of "conspiracy" and "theory" as individual words. But it's also completely false with respect to what "conspiracy theory" means in actual use - which is to group the subjects (here: people believing GPT-4 quality has been degrading over time, in spite of OpenAI strongly implying otherwise) in the same bucket as flat earthers, vaccine denialists, UFO believers, NWO fearmongers, etc.
Calling the belief "that the new GPT-4 quality had been deteriorated" a "conspiracy theory" goes beyond claiming the belief itself is wrong - it's also claiming that holding this belief implies significantly compromised reasoning skills. That is, it's just a drive-by insult.