Flying Dutchman dude.
Are you sure? I say you are posting an assertion based on price (or cost of goods to Apple).
My take is that replacing a 16-in MBP battery demands, at least relative to Genius "repair skills" and "bench time", swapping the complete upper case, together with the glued battery, (riveted) keyboard and speakers. Possibly only saving the touchpad, but only on the 16-in.
How do I know? Because I have done one such battery upgrade on a prior-generation mid-2015 MBPr, and it is major, difficult surgery. (There is little difference in layout between the prior-generation and the 16-in MBPr)
There is no way that Apple corporate would allow such extreme surgery on a customer's MBP on the reduced confines of their "clean room" backroom, with the uncertain micro-skills of the so-called Geniuses.