On normal tubes, the valve collar may, or may not have a small thin lock ring on them to hold them straight against the rim, or at least I think that's one of there uses! There not completely essential to a successful set up.
The valve collar on tubeless set ups has a larger, beefier lock ring on it with an O ring nestled inside to prevent air leaks. Look closely and you will see the edges of the O ring nestled in there. I strongly suspect the mechanic will have noticed that and correctly deduced that you were running tubeless.