Yes, they can; they can have the older app say "you need to upgrade your app" (likely with a 60 or 90 day grace period), and put the new one on the play store. Alternatively, maybe their native client already falls back on an HTML5 alternative on a cue from their server - I have no idea.
What is clear from this debacle, though, is that Microsoft was unable to write their app according to Google's guidelines, and then took 3 months and still couldn't fix it - which means that Google definitely cannot expect them to upgrade to newer APIs ever - so it doesn't seem onerous to require them to actually use the official API now.
What is clear from this debacle, though, is that Microsoft was unable to write their app according to Google's guidelines, and then took 3 months and still couldn't fix it - which means that Google definitely cannot expect them to upgrade to newer APIs ever - so it doesn't seem onerous to require them to actually use the official API now.