Yes of course it does, the muscles are working and this can stimulate circulation and even adaptation.
The main flaw that stands out (not saying it's the only one) to me is that, much like randomly doing isometric exercise (like trying to tear a rope in half, military pressing your doorway) is that while in theory you might generate as big a contraction with all the benefits of measurable resistance training... it's not measurable.
I think measurable progress has a big benefit in that you know when you're moving forward and when you're moving behind. It's a method of biofeedback. Methods like tensing agonist/antagonist pairs (usually to show off biceps/abs for most guys) are good accessors for awareness or messing around, but if you want to get progressively better, you pretty much have to rely on instinct to assess how hard you're doing it. I think instinct can fail us here, maybe depending on psychology or our state of health. "Really hard" is much less for an undeveloped person with injuries than it is for a strong person in good health, yet 100lbs is the same for both of them.
An advantage to external isometrics (pressing the doorway) is you could in theory get some numerical feedback. For example, you could press a scale against the doorway to see how much force your arms are generating, or stand on one while you do it and the amount it reads minus your weight is the amount you're pressing (could set it to 0 if it has that option).
The problem with internal isometrics (agonist vs. antagonist, like tensing your bicep/tricep at various angles to make them pop out) is that, short of maybe some device to measure muscle activity or some kind of internal joint pressure sensor (I think they insert stuff like that in some low back patients to measure the effects of traction devices and how much pressure they suffer from in the vertebrae) I can't think of any way of telling how hard you're doing it. That's the big benefit to using weight lifting and stuff because you know the numbers and can progress based on altering them.
That's also a problem with other forms of self-resistance, like opposing limbs against each other (e.g. 1 arm's elbow flexors vs. the other arm's elbow extensors). Unless you have some kind of machine (like squeezing a scale) to give an idea of how much you're doing, progress is hard and it's better as an instinctive awareness training accessory.
I'm not sure which would be better, like I think opposite/limb stuff would be better for getting a maximal contraction out of either because the joint is focusing on one action as opposed to 2 actions happening at same time. There might be issues like reciprocal inihbition that come into play and prevent really high-intensity stuff, though it probably fatigues faster since there's so much pressure in all the muscles (humerus/elbow in case of bi/tri) to impede lactate removal and decrease oxygen inflow.
