Why is the methoxide anion more reactive in a reaction with methyl iodide then the cyanide anion?

A faster rate for which rxns?

Methoxide is a stronger base than cyanide, and isn't more hindered, so I'd have put my money on methoxide in methanol as the "faster" nucleophile in a footrace. The substrate is about unhindered as a substrate can be, and the leaving group is very good. The whole notion is silly, though, since they don't yield the same product, unless you're considering using one versus the other as a base.

Chances are, that chart is a compilation of reaction rates from a "representative sample" of rxns, run in different conditions and yielding different products. Its purpose is to provide a "scientific" basis for what will rapidly become (hopefully) an intuition for what makes a good nucleophile and what doesn't, for which rxn conditions favor SN1/E1 vs SN2 vs E2.

If it bugs you, though, contact one of the authors and ask them to explain. (Yes, as a matter of fact I do.)