This is TTV staff’s (and as of recently, the contest auditors’) job to determine and enforce. If they find the modification to be within acceptable boundaries — as they did when they reviewed Helryx and Artakha — then it clearly falls within that rule. If the vetting allows that and you disagree with it, that doesn’t mean the vetting didn’t occur sufficiently.
To discuss the necessity of “consent system,” I believe it is beneficial to understand the history of the system. The consent system was formalized for the Hagar contest: a contest involving four MOCs with highly contentious features. Debates abounded regarding acceptable coloration for the Toa’s armor, whether the armor colors should be coordinated within the team, what variety of spearheads were considered acceptable, etc. TTV decided that the best way to simplify the contests would be to add a system of consent. That way, a voter could look at a MOC and understand whether that MOC could be reasonably expected to change color, spearhead, etc. or not. This allowed voters to, potentially, vote for MOCs based on what grouping of MOCs allowed for the color coordination, spearheads, etc. that they wanted.
This differed significantly from, say, the Artakha contest, where it was a given that the Kanohi would change and that the green on the MOC would be altered to the sand green that Artakha canonically has despite the shortage of LEGO pieces in that color. Consent to change the color or make minor aesthetic alteration to the MOC was a nonfactor because the existing rules of the contest already stated that such changes were not allowed.
If, say, a Helryx MOCist for some reason gave explicit permission to alter Helryx’s mace significantly, then the MOCist gave that permission. Otherwise, by default, it was understood that no such permission was given. No formalized system was needed because there was not such a contentious discussion surrounding the color coordination of four MOCs.