Trying to narrow it down to any one thing is unhelpful. The hands get you there before you can hear it, you can't say one is more important than the other. It takes a lot to even get close.I mean, that regardless of the fretless instrument, and regardless of whether one slides into notes or goes for dead on, and regardless of what fingering system one employs, the ears are what let you know whether or not you are in tune, not "Oh, it's Simandl, so I must have landed in the right place."
The remark about classical singers and fretless instruments is such a hyperbolic non sequitur and so irrelevant to the discussion that I'm not even going to address it.