I think releasing to this 3rd party so the internet can start chattering about it and discovering new functionality several months before an official release aligns with that goal of drip-feeding society incremental updates instead of big new releases.
OpenAI sucks at naming though. GPT2 now? Their specific gpt-4-314 etc. model naming was also a mess.
Maybe they got help from Microsoft?
* While lmsys does hide the names of models until a person decides which model generated the best text, people can still figure out what language model generated a piece of text** (or have a good guess) without explicit knowledge, especially if that model is hyped up online as 'GPT-5;' even a subconscious "this text sounds like what I have seen 'GPT2-chatbot' generate online" may influence results inadvertently.
** ... though I will note that I just got a generation from 'gpt2-chatbot' that I thought was from Claude 3 (haiku/sonnet), and its competitor was LLaMa-3-70b (I thought it was 8b or Mixtral). I am obviously not good at LLM authorship attribution.
The only case where detecting a model makes any difference is for vendors who want to boost their own model by hiring people and paying them every time they select the vendor's model.