Mostly personal experience, although
at least one other grinder mage player agrees. It's not that far fetched, honestly: control warrior struggles with decks that aim for extreme lategame as their hero power is effectively useless after they've used both shield slams and they don't have efficient removal for 2 mid-sized targets (which is all grinder mage needs to commit to actually win by chipping away at the warrior's health). Add to that that grinder mage can generate more cards via duplicate and they're definitely more likely to be having a minion stick in fatigue.
Control warrior simply can't win the value battle in that matchup and needs to be the aggressor, which it doesn't do very well at.