Hi Onion. Either your mate is full of it, no disrepect to your mate, or he is old school and hasn't kept up with developments this centuary
Fir instance, the Isuzu Dmax/MUX box is an Aisin box which is a jointly owned transmission builder with its transmissions installed in a number of different makes of vehicles. See here https://en.m.wikipedia.org/wiki/List_of_Aisin_transmissions The Colorado box is a GM transmission, very different.
If a transmission proves itself to be reliable other makes may use the same transmission and that is why your mate may think there is only one transmission maker however there are several.
All single turbos have a lag, dual turbos have a lag. Fact of life, nothing much you can do about it. Mazda has just patented, or applied for one, for a diesel engine with twin turbos PLUS an electronic supercharger to try to eliminate turbo lag. It looks like a really interesting development and should appear in future generations of the BT50. BTW, Maxda and Isuzu will be sharing their next ute platform and engine in their 2019/2020 model updates. Will be interesting to see where Mazda's new engine fits into this partnership.
As for the transmission being sealed, no it's not, despite what the manufacturers tell you. Yes, there is no dipstick to check auto oil levels. Yes, there is no service schedule in the service book. But yes they can be serviced. For my Dmax, Isuzu sell an auto filter. Why sell a replacement filter if the unit is sealed for life? I had my filter and fluid changed at 60K kms due to towing and many 4WD trips.
Heat is the killer of transmission fluid. If towing, add an aftermarket cooler and you won't have an issue.
KB