I think these are a variant on a concept (or gimmick) that Spiratone used long ago in filters called "Colorflow." (Took a while to dig around and find that name. Other companies also had similar things.) The two-color Colorflow filters had a pair like yellow/red and rotating the adjustment would change from full yellow, to intermediate, to full red. I believe the way these work is that the adjustment uses a pair of polarizers to blend between the two filters, although I can't at the moment come up with a fully coherent explanation of how that works. The Lifepixel link doesn't really explain it correctly. And of course they have a business interest in selling their own IR filters.
But I think Lifepixel's criticism is legitimate. You can get weird effects from the crossed polarizers, and the other issue that would concern me is transmission leaks. If the adjustable filter isn't perfect, you may get some light blueward of 720nm leaking through. Normally a small out-of-band leak doesn't matter for pictorial use, but when you are trying to use only the very red end of the film's sensitivity, even a few percent of blue leak may dilute the effect.
I would trust a cheap fixed-wavelength IR filter more than these things. Consider just buying two filters like a 720nm and either a Red 25 or 23. You will probably find that two filters covers most uses and you don't need continuous adjustment.