The depth of engagement doesn't matter in this case because the key is in shear across its face (.25" x .25", for example), between 2 components mounted side-by-side.

For 2 items fixed to each other by ID/OD, then the length of the key matters as it is in shear across the whole length (.25" x 1" long, for example).

Might not have used 100% accurate description, but the theory stands.

Consider that the camshaft gear uses only a small rollpin to locate it; the clamp force between bolt/washer/camsnout carries the load there. This fuelpump extender is a similar situation.