Here's the relevant tweet, from Ginny Caughey (@gcaughey)
Sometimes this message gets lost in the noise about why we can’t change things. Windows 10’s time has (finally) come! pic.twitter.com/CanRkApQSu— Ginny Caughey (@gcaughey) May 9, 2019
900 million + isn't bad for a desktop OS, bulked up by - as I say - Xbox mainly, plus a few phones. But it also has to be put into the context that many peoples' primary computing devices are their smartphones now. For example, I can think of a number of things which are faster to do on the phone because of the biometrics built-in, i.e. less need for fiddling around logging into things with a desktop OS and browser. Android is now up to over 2.5 billion users and iOS (iPhones/iPads) is closing in on 1.5 billion.
I suspect that sometime over the summer, with more and more businesses finally switching from Windows 7 (soon to be out of support), that Microsoft will announce a billion Windows 10 users.
Update: apparently the '900 million' in Microsoft's slide was a typo! It should have said '800 million' - a major typo, I'd have thought. Oh well. The actual amount is 825 million, it seems. Let's be generous and assume that it's OK to round up, eh?!