It's so odd that someone thinks there are "fake" switch motherboards that function exactly like the real thing, people can't clone game cartridges but they can clone an entire motherboard...ok.
But to answer you, the 12.0.2 fw came out May 2021, the OLED model released October 2021 so it natively shouldn't be possible to have that firmware on an oled model switch unless the boards were fully flashed around May. I don't have an oled so dunno if this would even work, but is it possible that you swapped the nand chip with one from a regular switch or did you not work on them when you did the board swap?
That's probably what the customer is worried about because if they just looked up the dates and assumed the system would be installed the day of release (which it obviously isn't), they'd expect oled switches to come with fw 14.1.2 since that was the latest fw before the oled release date.
Either way you should switch it back though, it's not their board and if something goes wrong later down the line you'll be held responsible for it because they'd probably suspect you swapped the boards "for a reason". That and if they ever took it to repair, even to another shop and not nintendo, it's possible they'd refuse to help the customer once they see the serials don't match.
