https://www.nbcsports.com/boston/patriots/how-cam-newtons-75-million-contract-fits-under-patriots-salary-cap
If I understand this correctly, that means -two- games of pre-roster bonuses count, because he played two games (which then falls under 'likely to be earned'), while any more than 2 games of pre-roster bonuses DON'T count against the cap because they're considered NLTBE.
So that basically means two games of preroster bonus plus his base salary is his cap impact for the year 2020. (That is, you need to count bonuses that -are- likely to be earned against this year's cap).
Which means, practically, this year, Cam Newton will cost 1.1375 million against the cap for 2020. They'd need to clear a bit more breathing room when spring training comes, but the amount of maneuvering they will have to do to clear room is not going to be that much. They'd probably cut an expensive OT backup who makes 2 million and replace him with a rookie free agent who makes like 125K and that'd be about it.
For 2021, the remaining per-game roster bonuses and any other NLTBE incentives hit then.
So, the roster bonus impact is technically correct, but badly applied to cover all the roster bonuses instead of just the 'likely to be earned' roster bonuses, and why pretty much almost every source indicates the Patriots' 2020 cap impact is around 1.15 million.