But where does Jagex state that BGH's primary purpose is for exp gain?

And even so, like I pointed out to another person, OP's BGH exp rates are right up there at one of the highest post-nerf rates as detailed in Jagex's modernization blog.

BGH is absolutely for money, the maul is a niche but useful weapon. It's going to have its place in an endgame PvM loadout. It's just that OP had some shit RNG and went dry on a maul piece. Going dry at BGH is comparable to going dry on Primus which fucks up your entire profit at Legions. Yah it sucks when it happens, but it's way to early to draw conclusions (especially not knowing the drop rates). BGH isn't like Magister where the variance on kills/khopesh is so small due to the double RNG mechanic. It's make or break and you just have to deal with it. Don't hunt if you don't want to risk losing money.