I didn't do a deep dive into the specs, but a quick check of their website seems to indicate this is a tuned antenna array designed to focus the transmitted (and probably received) RF energy - it does NOT seem to be amplified. As such, I don't think it violates any FCC rules; you can tweak antenna designs and even the simple slip-over parabolic reflectors do essentially the same thing but are not as sophisticated.
Using an amplifier to boost the transmit power is technically illegal but I've never heard of that being even reported, much less enforced, for drones. But still..................
I'm not an FCC rules expert, attorney or antenna engineer, nor do I play any of those on TV, and I'm only considering rules in the USA - so there's your disclaimer.
I don't see any legal issue with using the 4Hawks antenna mod, but I'm pretty sure it would void your controller's warranty by opening it up. Note that antenna tuning like this would make the signal much more directional (that's sort of the point), so you'll need to pay attention to antenna orientation toward your Mavic for best signal and to get the most out of the mod. The stock antennas on the controller are pretty much omnidirectional in a doughnut-shape around the vertical axis - pointing them flat and parallel to the ground improves the connection to a Mavic directly far overhead for that reason.