There is never an *either–or* in chemistry; it is typically a *both* instead. In the case of a hydroxide ion approaching an aldehyde with an α-hydrogen, it can *both* deprotonate the hydrogen creating the corresponding enolate *and* attack the carbonyl nucleophilicly to generate the required intermediate for a Cannizzaro reaction.

Most of the time, if there are two (or more) possible pathways and neither of them is intrinsically less hindered due to a strong reason, both pathways will be taken.