No, the DMA has nothing to do with RCS, at least not directly.
Yes, WhatsApp and Facebook Messenger were initially just XMPP servers, but have since had proprietary expansions.
In the context of the DMA both of them will be forced to make interoperability with other third-party messengers possible, since Meta has been designated a "gatekeeper" under the new regulation. Meta has announced they plan on doing so via an implementation of the Signal protocol.
There are some provisions in the Digital Markets Act that go in that direction. It only took effect roughly 3 months ago though, so remains to be seen if it works out as intended.
What the fuck kind of argument is this? Courts aren't supposed to play politics, they are supposed to enforce the law. And if you want to do that in a genocide case you have to prove intent. Gallant made several public statements that can interpreted in that way.