Kumble relied on uneven bounce, not spin. A pitch can have uneven bounce and not support much spin. On the other hand, a pitch can support spin and not have uneven bounce. Utilising uneven bounce is not related to utilising turn which a pitch offers.
No, indeed. However, for a bowler who spins the ball (which Kumble does - just not all that often using sidespin) uneven bounce + spin is better than uneven bounce with no spin or spin with no uneven bounce.
I don't agree that a spin-receptive pitch makes no difference to Kumble, as I say. He is a bowler whose stock-ball is the topspinner, but who varies the amount and sometimes bowls Leg-Breaks and Googlies. Hence, on a spinning pitch he's going to be more of a handful than a non-spinning pitch. And he
is going to be a handful even on a spinning pitch of true bounce.
His performances in last 4-5 years prove you wrong. We would mean normal pitches though, not flat tracks where almost no one usually performs.
That's the whole thing of a flat track, though, that almost no-one usually performs. If we take the start of Kumble's renaissance as that famous SCG game where he took 12-279, here are the games that stand-out:
8-172 at Multan - very flat pitch, of course, I don't really remember much of the Pakistan innings though
5-71 at 'pindi - same thing, really
13-181 at Chennai - super-spin pitch, of course
6-95 at Mumbai - same thing, even better in fact
10-161 at Kolkata - not sure about that one
Entire series vs SL in 2005\06 - again, not really sure here but I'd guess there was something in all the tracks for spin?
9-146 at Mohali - pitch had something in it for all bowlers, but remained true in bounce
7-110 at Sabina Park - very bowler-friendly pitch in many ways
5-56 at Wanderers - likewise
4-99 at Kingsmead - not quite so, but there was still something in it for most bowlers
And in between these times there are some flatter ones where he's been less of a stand-out. As I say - this is undoubtedly better than he once was, but still, perhaps, not quite as good as might be supposed IMO.