Those that only run open source software like myself have no secure way to run Signal short of compiling every release by hand which is impractical. Moxie has stated he will not support anyone but his team compiling or distributing Signal binaries so third party signed builds via privacy focused app stores like F-Droid are out. All builds must also use Signal centralized servers even though that centralizes TCP/IP metadata, etc.
Not to mention you need to show government ID to get a SIM to use the Signal wallet for said private currency/messenger in 200 countries.
Secondly having a decentralized currency whose servers can only run on Intel machines with Intel SGX is a very centralized supply chain as well.
A single supply chain attack on Intel microcode or related SGX updates could run malicious code and game over for the currency globally? A government that sees MobileCoin as a threat could make Intel do this.
With a SPOF on the supply chain of the only client people are expected to use and another SPOF on the only hardware enclave people are supposed to use for servers... decentralized is technically true but not used in the same way as most other projects that use that word.
I will keep an eye on this experiment though, because there are some unique ideas here which could have value should your trust anchors expand beyond Intel and Signal.