It is probable that CASA forced that advert. The 217 check and training manual should specify a minimum ratio of checkies and trainers according to number of pilots. Operators will run below these minimums as long as they can, but eventually an eagle eyed auditor will call it in. Depending on the size of the organization they can require these people to be full time employees, not contractors.
A salary of $150k is a big overhead for a Metro operator, for what would be mostly administrative and training, i.e. the checkie probably doesn't get to do too much pure revenue flying. But that's the price of doing business at this level of aviation.