The lack of availability in ChatGPT is disappointing, and they're playing on ambiguity here. They are framing this as if it were unnecessary to release 4.1 on ChatGPT, since 4o is apparently great, while simultaneously showing how much better 4.1 is relative to GPT-4o.
One wager is that the inference cost is significantly higher for 4.1 than for 4o, and that they expect most ChatGPT users not to notice a marginal difference in output quality. API users, however, will notice. Alternatively, 4o might have been aggressively tuned to be conversational while 4.1 is more "neutral"? I wonder.