Calling something a conspiracy theory is not an insult against anybody. It's a theory because it's unproven and it's a conspiracy because people think OpenAI purposely degraded their own service, hence conspiracy theory.
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.