Thats a pretty old calculation that doesn’t take into account hardware acceleration, which modern chips can do very, very well.
So you can get away with a celeron class intel chip (or whatever they are calling them these days) and mange multiple 4k to 1080 transcode streams without issue.
I have a mini PC with an Intel n95, which is around 5500 on the passmark chart, but can easily churn out 10+ 1080p to 1080p transcodes if needed, of course the ideal setup is to avoid transcoding wherever possible. that’s not my main server, but it’s nice to know that if I wanted to I could move my plex server onto it and it would be fine.