I mean, I have a "true" backglass sitting next to me as I type. Abra Ca Dabra by Ron and Nancy Webb. I count at least 9 or 10 colors on it, actually maybe 11, plus a blocking layer, but either way, it's plenty. No CMYK nonsense, these are individual screens. It was like $273 shipped.
Stern is using 5 colors. Cynan, Magenta, Yellow, Black, Mirrored. So whether or not they're using that ink on some kind of diffuser layer that they then bond to a piece of glass for a smooth finish, or screening directly onto the glass doesn't really change a whole lot as far as costs go. $400 either way is inflated. It just is what it is.
My Metallica backglass is mirrored, super flat (no little waves or ripples like a translight can have even when taped at the edges) and limited. I honestly can't say I care how it was made so long as those qualities are there.
BG Resto does the same thing. Would I take a Webb "true" backglass over their process? Sure, they're not screening each color separately, so it won't look quite the same. But Stern was never doing 11 color screens, so in this case it really doesn't make a difference either way.