It is ridiculous but you have overlooked the fact that the can itself holds more than 100ml and going strictly by the regs, it is classed as a LAG.
It has nothing to do with the amount of fluid inside, just as it doesn't matter whether a 150 gm tube of toothpaste is half full.
The screener was just following the letter of the law.
Plenty of us do the same, not saying it's right, just they are not in the wrong either.