Who said it was a YJM Signature J-Bass? No one. He's making a new album and was just showing off a really cool J-B.
Yngwie has several vintage Fender Bass's and he uses them quite often. He plays bass on most of his albums and he is very very good at it. I e seen him play an old Tele Bass and a fretless P-Bass. I does use a pick tho.