I don't think it has anything to do with mobile vs. club. It really depends on the DJ. I am 99% a club DJ, but I CAN do mobile gigs just fine. In fact the ones I have done, I have totally rocked out. A lot of them were not weddings, but school dances, parties, etc.
I think what they were trying to say is that most mobile DJ's don't mix, and in a club environment you HAVE to. They would be totally lost. In most of the clubs that I have DJ'd in, even a lot of "club dj's" would be lost, just because they were on another level of what you need to do to keep the crowd. You NEED to get on the mic. You need to know the hot jams, the old school jams, and especially the local jams. That could mean anything from local artists, to album cuts that the crowd thinks is hot. I was playing "Back That Ass Up" for at least 6 months before it was even thought of as a single. Most mobile DJ's can not come into an environment like a club and mix at the level you need to mix, and most club DJs are not able to make the transition into NOT mixing, and knowing what songs to play for a retirement party, or a wedding, or a corporate party.
Like I said, it really depends on the DJ more than any other factor. Sure, there is a certain style for each group, and one doesn't usually translate well to the other, but a good DJ should be versatile enough to handle both types.