Why not pick a mountain where you install the bunker 300m under the peak ?
The way I understand it, is that they drill horizontally, so it doesn't really matter how high the mountain is, but it does matter how high it is for protection obviously.
The assumption is that they haven't done that -- but it's not implausible to add some multiple of 10m on to the estimate.
I'd imagine they studied bunker buster arms in the design, and very probably concluded, that there wasn't much need to go very deep. Demolishing 80m of granite alone is a nuclear-sized problem, +20m and maybe 10m of specialised concrete, i'd imagine is fine.
It's also highly likely that the design of the installation is robust against collapse, eg., designed so that small areas can collapse independently. So even with arms which could penetrate that deep, you'd need a large number.
I think it's plausible that the entire supply of bunker busters the US currently has could do the job, but I highly doubt the US would risk depleting its capacity on a "maybe" of this kind.
The whole operation was a performance to try a carrot rather than stick approach with israel
That would be beyond any reasonable doubt that it cannot be destroyed (even with nukes ? ), which would make more sense to me. And the country is surely big enough to find one suitable for that.