Harkness test specifies that non-verbal communication is allowed, as long as both parties are fully aware of the question being asked and understand the implications.

Pokemon are capable of understanding complex commands that require knowledge of what the words being said actually mean. Even ones that don't talk, like 98% of Ash's Pokemon (the other 2% is the Rotom in his Pokedex, which is a Pokemon that normally cannot speak, but can do so when provided with a voice synthesizer).

EDIT: (example: training Froakie to climb up an enemy Rock Tomb, and then subsequently, in real time, because he didn't know the opponent even had this move, telling Pikachu to adapt the strategy by using Iron Tail to climb up an enemy Draco Meteor)

With both of these factors in mind, every Pokemon passes the Harkness test except for the 19 that are not sexually mature members of their species, which is the other Harkness Test criterion.