The Finite charm was trained to be used en masse by an entire army. It's a brute force spell requiring lots of power to dispel it's opposing spell. The usefulness of the sunlight potion wasn't in it's raw magical strength, but how quickly it disabled it's opponents.

So Harry had retrieved his copy of Magical Drafts and Potions, and begun looking for a safe but useful potion he could brew in the minutes before the battle started - a potion which would win the battle too fast for counterspells, or produce spell effects too strong for first-years to Finite.

He entertains either option, but he chose the more risky one that immediately finishes the battle. It merely needed to stand up to a handful of Finite spells, rather than a massed and coordinated dispel. I say it is the more risky one because he did in fact lose by choosing this option instead of brewing an invulnerability to sleep potion. If he could have chosen to make potions of any potency, he would have obviously chosen a certainly victorious spell of a risky spell.

The other critical limitation on potions is that you must known the stirring pattern and the recipe in general. Figuring out the stirring pattern is the sort of thing that gets you permanently turned into a cat.

This is evidence towards him putting in the magic himself. In order to deduce the stirring pattern, he looked up a potion with the similar ingredients and the same spell function from a preexisting recipe. If potionmakers could make the same potion using non-magical ingredients, then why wouldn't any of them have already invented a potion with nonmagical ingredients unless there was a significant drawback?