As far as I know, the Signal protocol was chosen to be the intermediary layer between the existing platforms. Matrix protocol was a candidate, but was not chosen.
Still, if you wish to use a bridge between Matrix and some other platform, then you can either set one up yourself or try to find a Matrix server that already has such bridge set up. https://matrix.org/ecosystem/bridges/
and that is why we hide our power level
https://www.pcmag.com/news/meta-adds-interoperability-with-signals-tech-for-whatsapp-messenger-in
https://techcrunch.com/2024/03/06/to-comply-with-dma-whatsapp-and-messenger-will-become-interoperable-via-signal/
I am still interested to know the details of how they came to this decision. Why Signal instead of Matrix.