I work in a shop full of professional mechanics, none of whom ride fixed gear bikes besides me. I took out a cannondale track that had been on the floor about a year (built by someone else in the shop) for a spin around the block, and exactly the same thing happened: a moment of distressing "coasting," followed by the cog spinning forward again when I stood and cranked hard on the pedals.
There is a degree of tightness that takes quite a bit of force to achieve, one which your mechanic might not have known was possible or necessary. One method I'm fond of is zip tying the chainwhip around the cog (to keep it from popping off) and using a deadblow mallet to tap the cog just a hair tighter than I could easily get it with my hand. This is not a pounding procedure, but the mallet lets me get it nice and tight without putting all my weight on it and busting my knuckles on the spokes. I do the same thing with the lockring, and it seems to be sufficiently tight to prevent what you're describing here.