Pffft, I have a Surskit with Surf in my Diamond PC that I got via GTS, and it hasn't caused any problems. Hacked Pokemon don't mess up or glitch your game.
Obviously this guy must've loved Swinub, and was willing to give up a Pokemon he didn't want / like (In this case, Magmortar). I've done it before, to get a Dratini, putting my Heatran up for trade, hahaha. xD