Whether it would actually improve strength significantly would be dependent on how strong or weak you were to begin with. What is does do a good job of though, which JT points out in the video, is teaching your body where it needs to be/how to “feel” the correct alignment of your musculoskeletal structure at the point of impact.
I actually will use this as a very quick way to correct a student’s positioning if I see their rear drive/brace leg is improperly positioned, they are leaning backwards at the spine/too upright, have their forearm out of the line of force, etc… There is a very fast learning curve with this as it gives immediate feedback to let you know if you are in the right position or not and students can feel the affects from adjustments that I make to their position immediately.
I actually like a method we use even better though where you start in your stance, have a pad holder place the pad against your fist, then have to push through the correct line to execute your punch (can be done with any punch). This really teaches you the correct alignment for the punches very quickly, or at least teaches you how incorrectly/inefficiently you had been executing them.
Then of course you need to have the pad holder hold the pad at a greater distance and work on your velocity, footwork/mass transfer, etc… so that you are punching and not pushing. But once you have the right alignment down your power will increase drastically, even at closer ranges/shorter distances of travel.
Again though, whether either method will make noteable increases to your actual muscular strength I’m not sure of. If you did enough volume, then it should as the body/muscles only really know whether they are working against resistance or not, they don’t care what the form of resistance is. Isometrics have been around for a long time though.
Bruce Lee actually used them fairly intensively during one point in his training lifetime and believed they helped him build strength and speed. Powerlifters, strongmen, gymnasts, and all sorts of other strength athletes also use them to varying degrees, so there is a decent amount of anecdotal evidence to support them. It is also a very movement specific strength that you are building, so if you did increase your strength it would likely carry over well I would suspect.