I can't find the original wording from them, just the german article from netzpolitik. I don't remember if it was just a press release or a public announcement.

https://netzpolitik.org/2022/digital-markets-act-sichere-messenger-threema-und-signal-sind-gegen-interoperabilitaet/#!

The important part roughly translated:

Signal's goal is to provide private and secure communication for all. Collaborating with iMessage and WhatsApp would ultimately degrade the privacy of Signal and its users. Other apps that do not have the same privacy standards as Signal would have access to large amounts of user data. This data could then be used or sold in ways that are not consistent with Signal's mission and values.

I mean, it makes sense. People move to Signal specifically to avoid their data going to Meta, so what would be the point in using it to talk to WhatsApp?