My iPhone 13 mini serial number starts with X, what does that mean?

I just got a refurbished iPhone 13 mini from reBuy. The serial number starts with X4 which is really weird, because the internet says when it starts with X, it was manufactured in 2018, which can't be possible right? When I look it up in Chipmunk, I get an error saying that serial number does not make sense. I just want to find out how old it is, to get some additional insurance.