The dealer using the diamond tester is a complete idiot, he doesn't know how to use the device, it wouldn't have worked even on a sapphire from the gen the way he did it.
I have the same diamond tester, and the manual is easy to find online, here's how you are suppose to do it :
Those testers works by checking heat response of materials, to work they have to warm up when you start them, there's even a indicator led for that matter, you can clearly see on the video that the guy just start up the device, put the threshold to 6 already and don't wait for the lamp to be ready.
It's only when his "test" is finished that the ready lamp light up ...
To use it properly in a average room temperature environnement, you have to start it, wait for the ready lamp to go red, then adjust the threshold to around the 3 to 4 limit, in theory you should even calibrate it on a known diamond of a known carat value to be very precise, but for sapphire it's not really needed.
Glass will never go past 4 that way, any type of glass. And Sapphire depending on the purity will reach the 7 to 8 limit at a reasonable speed, but not instant like a diamond would