Cycling pros regularly have VO2 max values of over 80. Kipchoge's is estimated to be close to 90.
Concerning the validity of the Polar Running index:
https://www.bisp-surf.de/Record/PU201408007535
Yes , world record material can go up to 90, I'm only talking about people nearing pro level, near 70 is already good enough for that (it's multiple factors though, with vo2 max only being one of them)
It's also much easier for cyclist to build up a higher VO2 max as they're less injury prone and can cram bigger volumes than runners.
I can't say much about the study, there are always a lot of factors to take into account before vouching for how good it is or not, sample size (18 is not a lot), sample quality (what does '18 moderately to well-trained male long distance runners" mean?), tests conducted, who ordered that study, how relevant it still is today (seems to be from 2013?).
Correlation does exist, thankfully, but accuracy probably vary a lot depending on conditions we know nothing about.
To have a fair evaluation of Polar (or Garmin, or Tomtom or whatever other watch manufacturer) algorithm they would need to be made public which is something they obviously won't do.
