Ah, now that's interesting, I hadn't noticed Law 19.
So Law 19 contradicts Law 16 !
So now I understand what's going on in this thread ! -- some of us are reading Law 16 while others are reading Law 19.
What a mess.
Can't believe it has taken us a year to notice that!
I think you have to do a lot of reading in context, but you can avoid a contradiction.
I'd have said law 19 is intended to be a quick summary of the full law set out at 16.18, and therefore should be read in context of that; I'd have thought the majority of the time a maul forming from a kick in open play will be because the non-kicking team has caught the ball and is immediately swamped by the kick-chase, with catcher's team mates attempting to help out or at least not lose masses of ground. In this normal scenario, the summary is correct. It's only in the rare event that a kicking team manages to catch its own kick and then is immediately engulfed in a maul that the full wording of 16.18 makes clear the kicking team don't get the put-in.
Equally, despite it only being mentioned in the part of law 19 concerning an unsuccessful end to a maul, and not repeated in the section concerning an unplayable maul from a kick in open play, the proviso as to what happens if the ref cannot decide who had possession at the start of the maul must surely be intended to apply to both - and that seems to be the basis on which Garces made his decision.
To my mind, that would seem the most sensible way of interpreting the various provisions. Although it would mean that if your opponent kicked the ball into your half of the field, and you and an opponent go up to claim the catch and you both get hands on it, you would be better off ceding the ball and then wrapping the ball carrier up the moment he hits the ground with a bid to form a maul then, rather than continuing to wrestle for possession.