VERY impressive, I watched the 15 min run and made a few observations. Using a cast refractory shell is genius, but not for something you hope to fly
Note at the start of the run, he forgets to take the starter off and the turbine temperature rises rapidly. One reason it runs so cool is that virtually no thrust is being produced (a stiff, hot breeze YES). Once you add an exhaust cone to the engine it increases the case pressure and temperature to get the thrust. Part way through the video he see's an increase in rpm. I suspect this was heat soaking into the rear bearing and the shell expanding allowing the race to have more play. You certainly hear a whine later in the video as the bearings are getting cooked, and how quickly it slows when the engine is shut off.
For any potential builders note, he ran this on a cold overcast day, this really helps the engine as the air is both cold and probably damp, it will keep the compressor cool if this is a printed component. I suspect the engine idles around 30k rpm or a little higher. It's still fantastic to see and a worthy build project. Just be EXTREMELY careful not to overspeed or overheat the turbine as those blades will leave the engine like shrapnel. On model engines the nozzle guide vane shell (surrounding the turbine) is stainless steel and designed to contain a turbine bursting, even something like this should be treated with respect.
I liked the Kapton tape for the compressor housing to case seal. On a model engine for flying you can easily see 120℃ at the compressor as it's working, it squeezes a lot of air very quickly
Just watched him doing a speed test and engine idles at less than half the speed I guessed. Things did go pear shaped around 40k though... Very impressed his compressor survived considering it's PLA.