Well, by calling it 3.5, they are telling you that this is NOT the next-gen 4.0 that they presumably have in the works, and also not downplaying it by just calling it 3.6 (and anyways they are not advancing versions by 0.1 increments - it seems 3.5 was just meant to convey "half way from 3.0 to 4.0"). Maybe the architecture is unchanged, and this just reflects more pre and/or post-training?

Also, they still haven't released 3.5 Opus yet, but perhaps 3.5 Haiku is a distillation of that, indicating that it is close.

From a competitive POV, it makes sense that they respond to OpenAI's 4o and o1 without bumping the version to Claude 4.0, which presumably is what they will call their competitor to GPT-5, and probably not release until GPT-5 is out.

I'm a fan of Anthropic, and not of OpenAI, and I like the versioning and competitive comparisons. Sonnet 3.5 still best coder, better than o1, has to hurt, and a highly performant cheap Haiku 3.5 will hit OpenAI in the wallet.