You must be thinking of a series other than Formula E (or at least, it's not what I'd call "huge success"). All of their current drivers had pretty normal feed

You must be thinking of a series other than Formula E (or at least, it's not what I'd call "huge success"). All of their current drivers had pretty normal feeder series careers (often including brief, unsuccessful stints in F1) and I don't see any that have notable sim accomplishments. WEC has made some amount of noise about hiring sim drivers (including a tie-up at some point with Nissan and Gran Turismo), but the same applies for the drivers in their top two classes. Even your source doesn't care to mention any drivers who have gone from sim racing success to what anyone paying attention to motorsport would call "huge success" in top-level racing (two of the GT Academy drivers did have a successful one-off drive in LMP2, with a factory team and a much better-established teammate).