In thirtysomething years, I have replaced dozens of water heaters and often there's so much sand in the bottom of the tank that the drain is completely plugged and the water only trickles out. In those situations, you have to make more holes to get all the water out. The thermostat on a gas heater is one of the openings that you can get to fairly easily to effect the draining.

It sounds like he was in a rush so, cutting the thermocouple to free it from the thermostat (so the thermo can be unscrewed out of the tank) would be easier than unscrewing it. Unfortunately, it's irreversible if you run into other problems.
Your plumber was headed the right way to complete that job but, shouldn't have subjected you to a loss of hot water because of his lack of planning/foresight.

(edit) That photo looks like he cut the gas line to the pilot, not the thermocouple. Also, the handle is missing from the drain so, there's a pretty good chance that drain is somehow screwed-up too.

Still, that's no excuse leaving you with a cold shower. These days, it seems that level of service (piss-poor) is becoming more and more common in the trades. One thing: in all the WH swaps I did, I NEVER left a customer w/o hot water and believe me, I had some pretty nasty swaps.

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Never a dull moment at Berry Hill Farm.