Earlier quoted context omitted.
Chat GPT is just GPT version 3.5. OpenAI released many other versions of GPT before that. In fact, Open AI became really popular around the time of the GPT 2 which was a fairly good chat model. Also, the Transformer architecture was not created by OpenAI so LLMs were a thing way before OpenAI existed :)
GPT-2 was not a fairly good chat model, it was a completely incoherent completion model. GPT-3 was not much better overall (take any entry level 1B sized model you can find today and it'll steamroll it in every way, hell probably even smaller ones), and the public at large never really had any access to it, I vaguely recall GPT 3 being locked behind an approval only paid API or something unfeasible like that. Nobody…
The first version of ChatGPT wasn't a huge leap from simulating chat with instruction-tuned GPT 3.5, the real innovation was scaling it to the point where they could give the world immediate and free access. That built the hype, and that success allowed them to make future ChatGPT versions a lot better than the instruction-tuned models ever were.