I didn't find that a particularly complete explanation - and the slot can't be closer to the CPU because? - I think it must be more about parasitic properties of the card edge connector on DIMMs being problematic at lower voltage (and higher frequencies) or something. Note the solution is a ball grid connection and the whole thing's shielded.

I suppose in fairness and to the explanation it does give, the other thing that footprint allows is a shorter path for the pins that would otherwise be near the ends of the daughter board (e.g. on a DIMM), since they can all go roughly straight across (on multiple layers) instead of a longer diagonal according to how far off centre they are. But even if that's it, that's what I mean by it seeming incomplete. :)