I don’t see any nails. I do see some things that might be either bracelets, or possibly could be ropes tying the hands to something, but no nails.
There are little dots that look like nail heads but they appear on the periphery of the figure. The whole thing is done in an ornate style.
Could someone with an incomplete or inaccurate account of the Crucifixion come up with a depiction like this? Maybe. The bible doesn’t spell out a lot of details about how Jesus was crucified, and I take it that other accounts of crucifixions have informed modern Christian ideas. One possibility is that the arms of Jesus were tied to the cross prior to nailing the hands (or arms).
It might have to remain a mystery until and unless more similar material, perhaps with associated text, is unearthed.