I'm actually going to go out on a limb and defend their choice of MobileCoin.
Signal has always been a decent privacy/convenience tradeoff and MobileCoin is no different. Features like phone numbers and contact uploading were always a user friendly tradeoff. They also seem to trust SGX.
Monero's isn't slow by any means but isn't fast either. UI changes can alleviate the issue but being unable to spend your entire balance until the next block is a step back in UX.
ZCash's lack of privacy by default doesn't fit with Signal's E2E by default ethos.
Using most privacy coins on a mobile device with limited storage is a privacy tradeoff no matter how you cut it.
The reality is that integrating a cryptocurrency for payments into Signal was probably the actual misstep rather than their selection of which token to use.