This is coming from Signal, who are more than qualified to do this kind of work. You shouldn't roll your own crypto. But crypto experts can do what they want.
it's good. think of it like adding a different kind of lock that requires a different key (method) to open up first. at worst it's no less secure than before. If it works as intended it's a huge disincentive for anyone collecting encrypted data with the hopes that a quantum computer may break encryption the "old" method in the future.