If the origin/destination is Wien rather than Wien Hbf, it's valid for the entire Vienna zone even when purchased with DB.

Sometimes DB issues tickets to/from Wien Hbf in which case it isn't valid from anywhere in the Vienna zone but you can still board the train at any station en-route. All tickets in Austria permit taking just part of the journey, e.g. boarding later or disembarking earlier. Thus boarding at Meidling won't be a problem.

Besides, conductors pretty much never check tickets between Hauptbahnhof and Meidling, it's a too short distance and they'd have to "recheck" the carriage again anyway for the new people who boarded at Meidling.