From reports it looks like the number is a combination of Nielsen Fast National data combined with streaming numbers via Adobe Analytics. My presumption is that CBS themselves crunched the numbers on that because no matter how you were streaming CBS - whether via Paramount+, via YouTube TV or some other way - you're still hitting CBS/Paramount servers so they have that information.
Obviously some people may have, say, started on P+, had a bad experience and switched to broadcast. But that'll all get averaged out in the data.
What's particularly important here is that they're providing a number who watched the whole game. It goes up to over 200m for people who watched a bit of the game. When streamers talk about numbers they can be anything from 30 seconds upwards of watch-time counting as a view, whereas a traditional TV number is an average of people who watched the whole show. So yes, more people probably watched the end of a tight game than saw the beginning, but it gets average out.
Personally, I made it until just after half-time (Usher isn't really my thing), and then slept because by then it was 2.30am on a workday... Woke up to the score but would happily have stayed up to watch it otherwise. Sky Sports took CBS commentary supplemented with their own presenters in a booth somewhere in the stadium. ITV used NFL Network branded pictures and had their own British commentator alongside a US colleague, again supplemented with presenters in a stadium booth - I assume right next door to Sky's booth.