I am not sure if I agree with their argument, and I know that other wallet software has had similar problems with malicious clones.
I don't agree with their argument at all. It is easy to create a malicious clone without access to the source code, simply by copying the GUI which is easily visible. You can copy the back end from an old version and just add a few lines of code to send any seed phrase, generated or entered, to yourself over the internet. It doesn't matter if your malicious clone doesn't have all the functionality of the real thing - by the time this is discovered it is already too late for the victim.
I don't see the claim on the download page in either the iTunes nor google play app stores.
They claim it here:
https://trustwallet.com/developer/https://github.com/trustwallet/wallet-coreThey are outright lying.