Yes it is, they actually use the signal protocol,[0] but they collect metadata which Signal supposedly doesn't (you can't really know)
[0] https://en.wikipedia.org/wiki/Signal_Protocol#:~:text=Severa...
[0] https://en.wikipedia.org/wiki/Signal_Protocol#:~:text=Severa...
Look at what data they can provide to governments when compelled by law: https://signal.org/bigbrother/