Yeah, that’s a great idea. One leg movements tend to really hit the quads more than squats themselves. They’re also great for strengthening the muscles that work to stabilize your knee and improve your balance. In regard to the article, it’s true alot of good athletes use machines (although I doubt they play a significant role in their training). However these machines have no carryover to any athletic event while squats are quite specific. A good time to use leg extension/leg curl would be if you’re really tired and are lacking the energy to do something like a unilateral exercise or a difficult Hip extension exercise like Glute Ham Raises (not easy, but worth the trouble). Then instead of risking inury it’d be better just to do a few sets of leg ext./leg curls (because they require little balance coordination and don’t tax the CNS) Although personally I’d just end the session right there.