Bubbles form when the dissolved air comes out of the insulin solution (outgasses). That will happen if the air pressure above the insulin decreases, or the insulin temperature increases.
When you fill a cartridge you’re instructed to let the vial reach room temperature before starting the filling process. The extra dissolved gas in the insulin will come out in the vial as the insulin warms. If you fill a cartridge with insulin below room temperature the dissolved gas will come out in the cartridge as the insulin warms up.
The remaining issue is any pressure changes in the air above the insulin. When you inject air into the vial before pulling the insulin out, the pressure above the insulin increases and more air will dissolve. When you withdraw the insulin the pressure will decrease and the dissolved air will come out of solution.
The temperature is the most important. As a practical matter injecting the air doesn’t result in a big pressure change and if you withdraw the insulin immediately after injecting the air, there’s not much time for the air to dissolve and diffuse through all the insulin.
Bubbles can also form via leaks in the system and the pump designs vary from one manufacturer to another so this might not be universal. My experience is with Disetronic (Roche) and Animas pumps. I just starting using a Tslim:x2 and it may have it’s own issues because the pumping system is so different.
I don’t think there’s anything wrong with what’s shown in the video. If it works for you - keep going!! But I would pay attention to the temperature first. I suspect during the time spent removing air from the vial the temperature equilibrates as well so it may be killing two birds with one stone. Also, degassing the insulin as they do in this video will probably make bubble formation due to any temerature increases as you wear the pump less likely (a very hot day or wearing the pump very close to your body).
If I sound like a geek it’s because I designed micro pumps for analytical chemistry instruments. Not the same as some insulin pumps, but the principles and problems with bubbles are the same.