I suppose they might have just wanted to whip something up using the equipment they had at hand, but you could easily solve the same problem with and off-the shelf device:
https://www.amazon.com/Perfect-Prime-4-Channel-Thermometer-T...
Which isn't any more expensive than the thermometer they used:
https://www.amazon.com/Digital-K-type-Thermocouple-Thermomet...
It sounds like OP was having fun. But I wouldn’t have bought a thermometer to throw away as an alternative.
Why? Time is valuable too. If I had to do the same thing and was more interested in getting the job done than building a Rube Goldberg machine, I'd totally buy a second thermometer that's suited to what I'm trying to do.
Of course, which is why it’s important to spend it learning new things.
You haven’t factored shipping into your time equation—get it done today (and learn something new) or wait 2 days for Amazon to deliver something I might only need once…I know what I’d do.
Thousands of dollars solution vs tens of dollars solution.