There is a lot to be desired about the rules for leagues. One that bothers me is that it actively discourages improvement (if you play your 10|0 games like bullet games, then I don't think clock management could be considered sandbagging, and now instead of winning ~50% of your games in 20 mins, you are winning ~50% of your games in say 5 mins as your rating stabilizes at a lower rating). Even ignoring things like that, there is no benefit at all to improving seeing as the only goal is to play often, and playing often is easier when you are expending little energy on your games.
To your point-- is playing simuls actually against the rules of the leagues or is it just frowned upon?
Well it finally happened. Someone is making good use of this strategy.
I was told that "technically" playing simuls is against the rules (even though chess.com fails to mention it in the rules).
One reason to make a rule against simuls is that only premium members can play multiple games at once... one reason to not put it in their rules is they have no idea how to stop people from doing it.
This guy is currently #1 on the crystal leaderboard
@marks1420
I checked his last 20 games. He played 20 games of rapid in a little over 1 hour... so it's roughly at the pace of 2|0 games, except rapid gives 5x as many trophies.