You didn't say that, I did. You said soccer kicks is not martial arts, but I contend that if elbowing a guys face to mush is, then soccer kicks are too. In fact any effective violence IS martial arts. The line currently drawn in MMA is the Unified Rules (more or less), One Champion and Rizin have differenr rules.
And absolutely it takes skill to land a soccer kick in a fight. If it didn't every soccer kick thrown in pride would have been some kind of cheat code to help end the fight, with everyone spamming soccer kicks, any time it went to the ground with the other guy standing it would be over...well it wasn't even close to that.... Its like any other attack, the guys good at it used it well, at the right time/opportunity. Yeah it might not ALAWYS take skill, (if the guy is practically immobile and finished), but similarly neither does dropping that falcon punch or final few punches/elbows to an all but finished fighter just before the fight is stopped.
"It just seems cowardly to you" is your bias, not logic speaking. its no more cowardly than punching a downed opponent or elbowing them.