If they’re rewriting Windows core bits with it, that’d seem a fairly strong indicator their involvement with it is serious. That’d be unnecessary technical debt otherwise.
Windows on ARM is probably fine. Windows Phone and the like eventually lost (losing? I haven’t kept up) support, but phones are traditionally a 2ish year supported device and then they’re not targeted (specifically in the Android OEM arena). Windows proper they have business clients and if they don’t support it for a reasonable amount of time, Microsoft is basically hosed. It is, I imagine, why once they decided to kill off older IEs everything prior to latest and greatest was killed at the same time: they had supported them for far too long and it required killing them all Off in one move.
Cool that MS is using something Ruat based. In no way does that opinion lead to a conclusion of ongoing support.