That is a very large leap to assume from the leaked data. IIUC the leaked data just says that there was a Signal account associated to his phone number. It could have been one friend or journalist that he wanted to talk to. He may have signed up to see what it is all about. As far as I am aware there is no evidence that he was a significant user, or that he prefers it over his own messaging apps.

I’m not saying that FB apps are trustworthy, but I honestly would have been more surprised if he didn’t have a signal account.