Yeah, the advisor mode does send the whole context, but you're only billed the cache input price for that, so either they just eat the cost, or they actually have something that makes it possible to move the cache between different models.

I have absolutely no clue how these things works, but could it be that all the newer models are actually just the same model, with looped transformers, so it's just about how many passes it does internally that decides if it's Haiku / Sonnet / Opus / Fable?

I might have misread the article a little bit, as you say, this is just workflow orchestration with subagents. Anthropic seems to think that this isn't the best approach anymore with their recent posts around subagent use and with the experimental advisor mode. So who knows.