Not true at all. The use of Signal started during Obama's first or second term. While the app's first release was during his second term, it existed under various names and forms way back. Wikipedia has a great article on its history.
I personally started using signal some time around 2018 and I'm sure there were millions of users by the time Biden began his term.
I thought the GP means that goverment officials such as the Secretary of Defense started using Signal during Biden's administration (though with no basis for that offered, yet).
Right. They also use cell phones and send emails. The difference is, from the Snopes article:
"... it explicitly did not allow use of Signal to communicate "non-public" Department of Defense information, which would have included the conversations Trump administration officials had in their group chat."
I personally started using signal some time around 2018 and I'm sure there were millions of users by the time Biden began his term.