I don't think people will confuse TMU station with Dundas West or St. Patrick. My argument is that, if they are not familiar with said acronym, they will have no idea what it refers to. Now, again, you can use Google Maps to find out, but you can also use it to differentiate between Dundas and Dundas West (not to mention engaging in step by step trip planning that precludes any possibility of one ending up at the wrong station to begin with), so any argument in favour of TMU as a name inherently validates Dundas and Dundas West, too. And so we go around in circles again, why did they need to rename the station in the first place?
Using an acronym is incredibly inconsistent with the TTC's pre-existing station naming guidelines. And for what reason? The cost savings of not having to print such large station signs must be trivial. I can't see a single compelling argument for why the name must be abbreviated. They're pointlessly complicating it for no reason, whereas utilizing the full name would ensure maximum understandability for all.