As I recall from one of your articles, you have the reach of an octave between 2 and 5. IMHO, that's exactly what one needs to do those exercises comfortably. I can somehow imagine what it feels like when one has (far) less than that, needs to overhold a 5-voice diminshed seventh chord and repeat the fourth finger on a black key. A recipe for disaster if you ask me.
Grip, however slight, is a co-contraction of both flexors and extensors. With weakened, already overstretched extensors, flexors cannot work properly to hold the keys down, so pushing with the arm onto the structure is the only option left. As I understood from his post, to simply repeat the notes as written, the OP has to make an effort to lift the finger to simply make the key come up to be able to sound it again. I seriously hope I misunderstood what he wanted to say.
If that's the case, as I suspect too, he'll do himself no good until he opens the hand in general, rather than strain to lift four. If the key can't push it back up, you're already screwed.
I've only just got that octave reach. I'm not much beyond average, if at all. I tried much larger stretches, for equivalence including a fourth between 4 and 5 while all other fingers had keys- and there was no issue with getting the fourth ready. The only issue that could cause that for me was a seriously squashed up hand with depressed knuckles.
I don't buy the idea that flexors hold keys down in these complex combinations, personally. I find its far easier when a sense of lengthening is the primary issue. This both expands the knuckles away, if you let the arm respond without stiffness, and gives a low effort means of getting clarity of contact. I feel a touch of inward grip sometimes, but I see that a possible add on rather than the fundamental action in simple over holding. I used to overdo it massively when I got these things wrong.
If you're using flexors notably, by the time the hand is open the action is virtually horizontal. That means you'd slip unless you dig in with the arm. A sense of balance through lengthening with an optional trace of gripping was what made it possible for me to do these kinds of things in a simple manner without any straining. Even if I do move the key only by raking back from the knuckle, the balanced state that follows involves a switch of muscle groups, not a continuous ongoing flexion. On that basis, not even the most extreme position my hand can reach would see me straining to get my fourth up, any more. Lengthening actions within an open position were the secret to my being able to do these kinds of things. They keep the hand open and the keys easily grounded without any of the needless down force that would see a fourth finger straining to lift up.
PS. I just tried now and it's very hard to lift any fingers when consciously keeping flexion in another. The indirect angle encourages arm pressure (to stop the finger slipping) which in itself stops the knuckles going up easily. But worse still, there's a physical conflict. I can feel I'm working against the actions that would lift any other finger ready to play. When you keep keys depressed by a lengthening, the action is actively elevating your knuckles in general. If the knuckle of that finger is going up, that of the next finger will be helped too. And if I choose to lift any other fingers, they move free of conflict in a way that links easily to the whole arm - all while the overheld notes stay clearly and simply down. I think flexion is everything you must avoid for success in overholding, except perhaps the faintest trace in the knuckle alone.
Topic: technique improvement