I think they mean allowing Signal to use RCS would let them combine all their messaging into a single app and/or use it for RCS on ROMs without Google apps. Kinda like how Facebook Messenger tries to be your SMS app as well, except with apps like Signal, people might actually use the feature.
If RCS is end to end encrypted, and doesn't use messaging fees, then every reason they decided to drop sms is no longer valid, except for needing to spend more time developing it.
I doubt they'll add it even if they did have the option, but it's not ridiculous that they could.
The removal of SMS will be necessary because Google is cutting out third party apps from being able to access the messaging framework, including RCS. Google's closing the walls around their garden.