It's rather that it's more effective at converting energy to impulse, since the molecular weights of the combustion products are much lighter. LH2/LOx doesn't

It's rather that it's more effective at converting energy to impulse, since the molecular weights of the combustion products are much lighter. LH2/LOx doesn't have a particularly large energy advantage over hydrocarbons, when you're also accounting the mass of the oxygen (as you do with non-airbreathing rockets).