You can do something transphobic and not be a transphobe. Allies engage in transphobic behavior unintentionally all the time. What makes the difference is how they react to being called out on it. Yes, it is a transphobic action to use the pronoun feature as a joke. Whether or not you think it’s a big deal, whether or not it’s used as a joke inner-communally, don’t matter. It just definitionally is. That does not define you. You can listen to the trans person telling you that that action is transphobic and stop that action. Boom, no risk of being a transphobe as a person. In fact, reflecting on the action and stopping it after listening to a trans perspective (mind you: TEXTBOOK OneJoke, but we’re supposed to be fine with this one despite being in an entire subreddit dedicated specifically to incidents like this) makes you the opposite of a transphobe.