Yep, it does seem that way. FWIW I was asking Claude yesterday about the relationships between these different "tier" Anthropic models, and it is not aware of any fixed relationship between them, so it's quite possible that Opus 5 and Fable 5 are related (or not), but that Sonnet 5 is not. Claude suggested that Sonnet 5 was just named Sonnet 5 rather than Sonnet 4.7 since this seemed more appropriate given it's performance relative to Opus 4.8.

OpenAI's versioning seems equally opaque. Claude mentioned that there was a tiny bit of clarity from them in that GPT 5.5 was the result of a new pre-training run, which would seem to strongly suggest that 5.6 (following so soon after, and given the choice of naming) is therefore based on 5.5 - but again who knows.

It seems that so much of the model performance is now coming from post-training that this is what is driving inter-version performance differences, and that base models are much less important than they used to be.