Wonder if they're referring to the two different knights, both which can take d4?
Repeated moves in Opening Explorer

yes, probably: Nbxd4 and Nfxd4. Play them both and see.
https://www.365chess.com/opening.php?m=17&n=1483&ms=e4.e6.d4.d5.Nd2.c5.Ngf3.cxd4.exd5.Qxd5.Bc4.Qd6.O-O.Nf6.Nb3.Nc6&ns=3.16.12.17.74.682.583.880.5560.1309.1232.1310.1464.1482.1465.1483
https://www.chess.com/explorer?moveList=e4+e6+d4+d5+Nd2+c5+Ngf3+cxd4+exd5+Qxd5+Bc4+Qd6+O-O+Nf6+Nb3+Nc6&ply=16
So I was browsing through the Opening Explorer, and I reached the following position:
And here I read white's top two choices are 9.Nbxd4 (with 2644 games played), and 9.Nbd4!! (With 1988 games played)
But they are both the same move!
Bug or what?