No bias obviously as the index hasn‘t changed a lot if at all but the label ’serve leader‘ isn‘t precise and if you synthesise a lot of factors into a single number you obviously lose nuance.
The insight algorithm gives Sinner the best forehand
and backhand in the game but only
the 38th best serve quality*:
In short Sinner has now the strongest serve plus in the game because there is no safe place for a decent return, as his follow-up is elite from basically every area of the court. His number 1 in conversion is the result of rarer attacks finished very efficiently.
GMP is by far the better server although we will how he stacks up in the long run against the best returners. We should see quite a bit of experimentation with the return position. As pure servers I would put over at least ten players very clearly above Jannik.
*He reduces - like Alcaraz btw - his serve risk against weaker base-liners so their values might understate their prowess.