It's even less than that. While technically against the TOS, Prime subs are often shared over several households (just like other streaming services)
100 million views doesn't equal 100 million accounts that watched the series. It's probably more people that watched it, but significantly less active subs. For the 9 people I know personally that watched the series, only one active prime account was used, divided over 3 households, so one sub accounts for 3 complete viewings of the series. I doubt that that is representative, but it's pretty safe to say that drastically less than 100 million accounts actually streamed RoP.