The number of bike mechanics that don't know how to install a track cog & lockring correctly baffles me.

The problem probably wasn't the lockring being tight enough. The cog first needs to be tightened down properly before the lockring is cranked on. If it's not, the cog will slip when you apply any amount of back-pressure.

Tough to tell from the pic but it looks like a couple of the outer lockring threads are a little messed up. There's probably still enough to safely use the hub though.