A shorted to ground compressor can't be a pre-existing condition can it?
I agree with Harper they are trying to rip you off . The unit worked when you purchased the home now a year later? They say there was a pre existing condition, I say BS! Compressor  oil can go acidic in a very short time, infact a brand new compressor installed in an acidic system that was not properly flushed and new filter dryer added can short to ground in just a few days.  What causes systems to short to ground? The leading cause is moisture getting in the system and mixing with the oil, once there is moisture in the oil it will go acidic and start etching the insulation off the wires once the insulation is gone they short to ground.  There is no way to say this was pre existing it probably happened from sitting all winter. I would fight them on this especially since you have had the home for a year, if it happened within a month I could see them calling it preexisting. Call another hvac company and get there opinion, (I do have a universal 608 hvacr license and a 609 Mvac so this is not an uninformed opinion).