Here's a completely made up point of view. (I haven't read the articles.) GPT-4 is not an LLM, but a complex software system, which has LLM(s) at its core, but also other components like RAG, toxicity filter, apologizing mechanism, expert systems, etc. "GPT-4" is product name / marketing name. For OpenAI, this would be logical for performance and business reasons. This explains also how they can tune it, the apparent…
The Assistants API is closer to ChatGPT & what you are describing.
Edit: it should be noted that the Assistants API is somewhat model agnostic as well, so the product part of this isn't part of the inference system.