That was certainly the case for a while - they closed the source for the Android app to try to prevent malicious copies of the app appearing on the Google Play store: https://medium.com/@trustwallet/why-open-sourcing-android-app-could-be-a-harm-to-the-crypto-community-fb3ae1707dc6

However, looking at their GitHub repository it looks like both the Android and Apple apps are now closed source:
https://github.com/trustwallet/trust-wallet-android-source - archived, last commit October 2018
https://github.com/trustwallet/trust-wallet-ios - archived, last commit December 2018

It seems that their underlying libraries are open source (https://github.com/trustwallet/wallet-core), but not the wallet apps themselves.