I don't think it seals with or without the dust boot. The boot goes around the outside of the cable, but the cable is effectively a long straw. So it's not sealing anything.
Secondly, the slide rises up into the body when you whack open the throttle. that air has to go somewhere. Probably a combination of out of the top of the carb, and to the intake side of the carb.
Thirdly, the slide is constantly being "sucked" towards the motor, so any play in the slide will be on the intake side of the carb, which is the "ambient pressure" side (actually at a light vacuum, the same as the airbox would be). I suspect the sealing surface is the motor side of the slide, and any "leak" on the intake side of the slide isnt a leak, just another port in the airbox.
Leaks would be leaks that are downstream of the slide, after the fuel has mixed.
(disclaimer... I like thinking about physics, but am not an expert on carbs or dirt bikes, so don't confuse my theory with fact... and go run a test before believing anything I say. :) )