Contrary to its name, "ultrafast" isn't faster than the rest, and many times slower than "max", as it'll "fork out" to a bunch of sub-agents and wait for them, + does extra "red-teaming" and more.
I think "ultrafast" is not referring to the speed of the "model" (harness in reality, as it's all the same model as "max") but rather how fast it consumes your usage limits.
I believe you refer to the "Ultra" mode that does what you say and it is also mentioned in the blog post, but I don't think the two modes are related.
Ah yes, I guess the portmanteau confused me and I assumed they were talking about "Ultra" the "reasoning effort" (which it isn't), rather than the "fast mode" which supposedly gives you priority over "non-fast mode requests". Although in practice, counter-intuitively, sometimes being in non-fast mode gives you faster replies than fast-mode, haven't got a feeling for why/when though.
He said "ultrafast" which has nothing to do with subagents. It's a new API tier where it runs on a different inference backend to get you faster token/s.
OpenAI really is the worst at naming stuff.