It's not either-or. You can use both.
If you prefer to switch apps and identities and go over to Signal, awesome.
If you'd rather message someone with your ActivityPub identity, you can do that securely now, too.
The E2EE work on ActivityPub uses an open standard, MLS, to encrypt data. One reason we chose it was so it's at least possible to bridge to other social and messaging networks while keeping the data encrypted from end to end.