I assume you are referring to the one titled 'Extrusion Mystery Solved' from like 3 weeks ago. I have no idea how I missed watching that one. I just watched it. His explanation makes sense.

I really wish I still had a lathe now. Shimming should be a makeshift option to better center the BMG drive gear.

I've been playing with a way to adjust both sides of the idler gear shaft like how an old CD ROM laser carriage linear rail system is adjusted and aligned. I only have that half done in CAD.

Now I am wondering about the effects of center tensioning versus independently adjusted sides. Plus there is the shaft/needle bearing clearance. It might be better to run a second set screw drive gear with shaft, centering shim, and cartridge bearings on each end of the shaft.