If the hole is not blocked off it will allow free passage of air from the uppermost parts of the carb.
If a cap comes loose or the cable does not make a good seal it could result in a lean mixture.
This is very feasible, but I was thinking, if it was open, it may allow better lubrication of the slide(maybe). If the cap or cable don't make a good seal than you will have a lean condition, no matter the hole. Of course you guys have tons more experience than me, so better to stick with what has been working!