Why are fast printed lines thinner?
In your case, since linear advance makes the issue disappear, the reason of the thinner lines is the same as explained in the link you provided:

The section (2) simply is very long because you probably have a Bowden setup which takes more than the length of the middle section to recover. However it does recover, see the first line (k=0) in the first image.
At higher (constant) speed the flowrate decreases further due to pressure in the nozzle causing a reduced supply of filament, but this effect cannot be corrected by linear advance because linear advance assumes a linear relationship between pressure in the nozzle and extrusion.