I guess the rules as written is cheating now…

Next you'll tell me when rolling character attributes you roll 5d6 and discard the lowest 2.

If you found this in the rulebook for any TTRPG, then yes, it would make sense to follow the rules within that game.

Advantage/disadvantage is very explicitly defined in the rules of D&D.

Did you play before 5th edition?

What? Having "advantage" is a specific bonus for that one roll that was triggered by something, as allowed by your current ruleset (and DM, obviously), where you're supposed to take the higher dice. Why assume that it's "cheating"? Lots of people play games that allow that in specific scenarios