In a competitive game mode, fairness is mandated by circumstance.
Can you imagine a starcraft II tournament where one player’s units were just inherently stronger than the other players?
In some cases, a really great player would be able to overcome a handicap, which is, at least to some extent, what Vanilla PvP was all about. Utilizing teamwork to overcome gear discrepancies and the strengths and weaknesses of the various classes.
That said, matchmaking pugs and premades together is like providing 1 player with more basic tech buildings to start the match. He’s getting marines out more quickly, because he doesn’t have to invest the time it takes the other player to construct his barracks.
If both players are provided a barracks at the start, then the game is still fair, and the competition isn’t invalidated; it’s just relying upon a different ruleset, where the players are inherently provided a barracks in addition to a command center at the start of the game, which ultimately changes the way the match plays out.
There are two separate rulesets which determine how a team is selected, one allows players to hand pick their own team, the other provides random teams. Premades facing pugs aren’t playing the game utilizing the same set of rules, which frequently results in stark power imbalances.
Is it possible your randomly generated team will have a druid, a priest, a paladin/shaman, a warrior, a rogue, a hunter, a warlock and a mage or two? absolutely. It’s also possible you’re going to get 8 warriors and a warlock and a mage, and in the random teams rulset, the objective is to still win the game despite not having access to some of the class mechanics which would make completing the objective of capturing the flag easier.
In that case, you might have the mage run the flag with the support of a single warrior and have the rest keep midfield clear; or provisionally have a warrior run the flag, pop shield wall, fear, swap carriers and continue on through to the base.
When your team composition is random, you have to be more creative in terms of how you intend to complete various objectives. When your class composition features every tool you could possibly need at your disposal, you’re playing an entirely different game.
So yes. You won, because your team was hand picked, while the other team was randomly generated. That doesn’t make you a better or more competitive player, it just means that the premises of the match were built upon circumstances which were inherently unfair to the other team.