If you go back to Notebookcheck you'll see they updated that figure with this disclaimer:
"Thanks to a readers tip, who told us that he is suffering from problems caused by flickering / PWM, we looked at the Core i5 version again with the newest firmware and drivers. Now we could also measure flickering at 50 Hz below 50 percent brightness without a connected power adapter. Analyzing our readings, it does not seem to be classic PWM but some other form of flickering."
No, you seem to have misinterpreted that update!
You are refering to the review of the i5 model (that I too had) on the english site of notebookcheck.
I was originally talking about a PWM comparision of notebooks on the german site of notebookcheck. (PWM Ranking - Die augenfreundlichsten Displays). It is easly understandable for non german Readers as well, since it is only a Ranking and the surface 4 is on top (worst).
The very high PWM value of 10420 though does actually come from the review you mentioned, but is only in the longer original german review.
However if you read the original review or the translation carefully (I guess something got lost in translation or you should just emphazise the word "also" when reading - i made the letters bold in above quote "we could also
measure") - it becomes much clearer that the reader's tip refers to an additional, different flickering problem on the surface.
My guess is it is the occasional very visible flickering you get. I also had this problem and actually could get rid of it by disabling Hyper - V.
However I still got bad eye strain from the surface 4 (the PWM), so I had to return it.
But I get that people can get confused over all the different flickering issues the surface displays.
I guess it is hard to relate how bad the effects of a PWM eye strain from severals hours on a display like the surface's are if you are not one of the affected individuals.
The only thing that - for me - is comparable to working on a surface is having to watch several hours of film on an active 3d television with shutter glasses.