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.