Well, "no GPT-5" isn't the same as saying "no new trained model", especially in the realm of marketing. Welcome to "GPT 2024" could be his next slogan.
That is one AI CEO out of 10,000. Just because OpenAI may not be interested in a larger model in the short term doesn't mean nobody else won't pursue it.
Others might pursue a smaller model that works as well as a larger model. If that can be done, whoever does it can very effectively compete on price/performance.
It seems that to assume otherwise (the only way to improve is to get bigger) is to assume that OpenAI already has found the optimal architecture. That seems unlikely.
Sam explicitly said that there won't be GPT-5 in the near future, which is pretty clear evidence unless he's blatantly lying in public speaking.