It's a matter of interpretation.
The law is stupid. It ties the legality of >10rd mags to their interchangeability between a rifle (legal) and pistol (illegal).
The reality is that any rifle receiver can be (and probably has been) modified or manufactured as a pistol. So, from that viewpoint, there are no legal mags >10rds.
Then comes the gray area. If there's no pistol in commercial production that you can legally buy at the local gun shop, or that can be purchased online and legally registered, then the existence of a compatible pistol is a false claim. Likewise, if the home-built pistol does not qualify as a semi-auto with a removable magazine, then the >10rd rifle mags are legal.
That's been the case for AR's. We can have AR pistols, but only if the thing is not a semi-auto and/or the mag is not able to be removed. Either of those conditions negate the mag's semi-auto functioning and/or mag removal, thus negating the interchangeability between that AR pistol and a semi-auto rifle.
Some local shops go with the technical interpretation (that a compatible semi-auto pistol with removable mags does exist -- just not in Hawaii) and only sell >10rd AR mags to cops. Other shops will sell to anyone until the law changes or a precedent is set making them unquestionably illegal for non-LE.
To be safe, some shooters only use 5rd or 10rd mags at the range. No biggie, since there's a 5-shot-per-reload limit. I use 10rd mags and some 20-rd mags blocked to 10 for range time. What I choose to leave at home may be larger capacity.
As long as the current law stays stupid, and the AR mag restriction remains unenforced as it has always been, the gray area will exist. They really should have been less cagey with the way they allowed some >10rd mags but not all.