Does anyone know whether there's any implication for Signal? Will they have to comply as well and make their app interoperable with other messaging apps?
No, because the regulation would only apply to market-leading "gatekeepers", with thresholds for revenues after which it would apply, so Signal are safe.
That's fantastic news, thank you!
There is a signal api already
Sure but that's completely different from requiring the Signal app to allow exchanging messages with Facebook/WhatsApp/iMessage/…
I read a while ago that the regulation will require them to build an API that other apps can access it, so you could use an alternative app if you want to. I'm not sure if they will force them to receive messages from other services.