He is jerking your chain a little. Condensing units are shipped charged....usually charged for 25' of line. BUT, this is to keep the system dry, and exclude debris. It is certainly NOT appropriate to just " plug it in a go". An installer MUST connect gauges to make the proper adjustments on charge for your exact installation. He does this with superheat set up if on a piston, and sub-cooling if on a TXV.
I would guess that they just connected your unit and never did the set up. This is sloppy. The charge doesn't just leak out, so now we have to wonder whether your installers were just careless, or if you have a leak/
Sounds like the guy you called in set it up, so if there is not leak, you should expect no further problems. I hope this service call didn't cost you anything.