That's in Mayhem to save time for waiting on abilities. The only way a hunter can throw out that many mines otherwise is with Young Ahamkara's Spine and hitting targets with the Weighted Knife, which needs a kill to recharge. The only way they can throw out that many knives is with exotics that offer multiple melees, refresh them, or have multiple dodges. Warlocks with the right set ups can spam abilities even more than hunters. Solar can have infinite grenades and recharge melees in like 2-3 airborne kills for example. Titans can get instant grenades that CAN OHK (Ashen Wake with Roaring Flames 2X), and HOIL can let you get back abilities in seconds if you cycle through them right, even abilities like Diamond Lance proc it. They also have an exotic or 2 that can basically give infinite melees. Hunters aren't really any different from them in this regard.