You shouldn't normally be able to put a foot down while sitting on the saddle. That was as true for me when I first got my hybrid out of moth balls 2yrs back, as it is with my road bike today. Riding at the proper saddle height can feel weird at first, but it's optimized for your foot being extended while riding not sitting with a foot on the ground. It's pretty well known that if you ride with a low saddle you put more stress on your knees that you need to, I proved that to myself once I started riding clipless, pushing harder and taking on bigger climbs.
If your riding style and saddle height cause you no problem though, good for you, but your mechanic wasn't BS'ing you.