It is. Monero community seems to have made so many choices that prove it is dedicated to the "digital cash" use case (dynamic block sizes, constant tail emission, etc).
Similarly dissapointed but looking at Signal's past choices (barely open source server, no plans to federate, aggression against modified clients, extensive use and lack of criticism for SGX) monero wasn't even on the table.
Using an established cryptocurrency doesn’t provide a good way for them to profit.
MobileCoin is pre-mined and the majority of coins are held by founders and the MobileCoin organization. They want to sell you coins, so partnering with Signal to force you to buy their coins to transact on the app gives them a revenue stream.
If they simply added Monero, they wouldn’t be able to sell you coins.
Only thing I can think of off the top of my head is optics (most of the installed userbase for signal has probably only ever heard of monero in a negative light).
Multiple reasons why MOB over monero but I will state only the most practical one, using the stellar consensus a tx takes 3-5sec. With monero that's more like 1-2mins if Im not mistaken which is a big no-no for the usecase of instant mobile private payments. Fees are also super low w/ mob (0.01)
XMR transaction can show up instantly once they are in the mempool, a user does not have to wait for a confirmation to see a transaction. Obviously there are uses where you want to wait for confirmations, but for the use case of sending money to friends it isn't necessary.
Using 0-conf is restricting usage to "friends only" which is a tiny subset of who you may want to send money too so I think you've already answered this.
Also moxie replied in his twitter about why not monero stating they wanted a "non custodial" integration, not sure what he means but you can ask him.
