Ah, I assume you mean this one....
Nice calc, haven't checked the maths but assuming they are correct, considering Sky & BT broadcast at 50 FPS & VAR are looking at the same feeds, the figures are out by at least a factor of 2. Slow motion camera's (of which Sky use several at games) are often 3-4 times that frame rate as well (150 FPS +). Agreed 2-3 cm would still be difficult to judge, but not impossible depending on the actual time the frame is taken and 10cm (4 Inches) should pose no particular problems. Don't recall seeing decisions of 2-3 cm though, the lines on the screen would overlap, not just be close...overlap.
In any case, considering the human eye sees at around 10-12 FPS (although it can detect movement at much higher rates - that's why strip lights appear to flicker, especially at the edges of our vision), even 25 FPS is much better than the human eye (i.e. the linesman) could see. By definition VAR cannot but help and improve offside decisions (penalty/red card decisions are a different argument).
Don't get me wrong, I don't think the current implementation of VAR is perfect by any means, but it will get better & faster.