Having become an avid viewer of BBC-4’s defunct “Time Team” archaeological series, I’ll venture that the dating is based upon pottery remains and possibly a few coins, in addition to the carbon dating.
Yes, that’s why I asked about the finds. Of course, one of the big problems with the Picts is that they left so few finds, much less datable ones. They did leave a number of rock and monuments carving, but with no written language, those are not yet understood. This site was thought to be Iron Age, but now is believed to be Post Roman and Pictish, but we’ve yet to see the evidence that supports that conclusion.