A “manhattan project” can just mean a massive secret scientific war project? Seems apt.

They have high security, and obfuscating the premises is part of it, but is it really secret in any way ? I mean, we're knowing exactly what they're aiming for and could compare notes at the end of it.

Is it war ? in a "everything is a war" political sense, perhaps, but not in any other sense.

We're left with "massive project" for the analogy, that's kinda weak really.

and a critical nation scale ambition

I think the Manhattan Project is a poor analogy. Moonshot is more like it.