That's all fair, and your hypothesis is pretty much spot-on in terms of what happened. The huge productivity boost was basically key to the success of the project.

I'm not sure whether the argument now comes down to "it would have been possible to gain that sort of productivity boost in C++ too, with the right design" or "it would have been possible to implement a new CSS engine without this sort of productivity boost". If it's the former, then you're right that one can't prove a negative. If it's the latter, then I think that's where the "schedule and manpower constraints" come in.