If we're saying that HD don't correspond very well to CR, then yeah. The aforementioned hippogriffs are glass cannons -- they have two fairly respectable attacks, and relatively few HP, so their "offensive CR" is way higher than their "defensive CR."
But the reverse can happen too. Summon up a bunch of rhinos or elephants or something big and meaty and have them harass the adult red dragon from multiple angles. If there's a large group of them, they become tempting targets for breath weapons. And absorbing two breath weapon attacks is usually twice as good as absorbing just one.
Besides, fighting a big solo dragon isn't the only scenario for using hirelings. Consider fighting a bunch of drow, for example. Would you rather have 10 guards (9 hp) or 10 berserkers (36 hp)? (I might be misremembering these HP numbers, but you get the idea.) The guys with good HP can form an effective battlefield presence and at least pin down some drow for some length of time. Maybe not 4x as long, but it is still probably better.
So I agree that you can't use HD as a proxy for "overall power" in 5e. But saying "ignore HD" might be going a bit too far in the other direction.