For 300pts? Yes.
But this 300pts surely encompasses not only the grenades, but the fact that there are five squads of infantry. each with lasguns and totalling 50 men. Now, I don't know if lasgun fire can take down a MC, but if you use all 300pts of troops to mob said MC, it surely won't be able to kill them all, and you would get 5 grenade attacks (as they're separate units), so you could potentially hurt the MC, otherwise you can keep it locked in combat and out of the game for a long long time.

It is also because "throw" was only used with the Shooting Profiles and "used" was the verb in the Assault Profiles.
I think this is where the original interpretation really came from, but when you read the initial sentence, the first thing in the Grenades section of the rules, it isn't much of a stretch to understand that it applies to both phases. The first sentence mentions both shooting and assault, and the second sentence clarifies that only one grenade can be used per phase. Sure, you can read each sentence in isolation from each other, but that's not how rules work in 40k or any other game.