There is very little empty space in a phone, so conformal coating is practically the same as filling it.

Anyway, I wasn't disagreeing, just reasoning a bit further.

The point of filling it is to remove the compressible empty space so that large pressure gradients won’t crush it.

No, conformal coating and potting are extremely different things done for different reasons.

I'm not talking about which methods are being used, I'm talking about which methods could be used. Further, potting, where you let the epoxy drip off, gives you a conformal coating.

Conformal coating is much less viscous and would leave a layer orders of magnitude thinner then letting potting epoxy drip off. It's not at all comparable.

You are talking about a conventional conformal coating.