They couldn't do it because they weren't fine-tuned for multi-agent workflows, which basically means they were constrained by their context window.

How many agents did they use with previous Opus? 3?

You've chosen an argument that works against you, because they actually could do that if they were trained to.

Give them the same post-training (recipes/steering) and the same datasets, and voila, they'll be capable of the same thing. What do you think is happening there? Did Anthropic inject magic ponies?