Hey, enby here. While I definitely benefit from they being a default I have enough binary trans friends who have this experience. What the person you are replying to is giving you is something referred to as nuance. A solve put forward by a well meaning cis person doesn't automatically work just because it seems like it should to you. Sometimes it causes new problems and when someone tells you about them it's a good idea to not assume it's them trying to be a dick or difficult about something but actually explaining why that solve isn't always a good thing.

If your intention is to make a trans person actually comfortable instead of getting defensive then listening when these things come up instead of telling them they are trying to be trouble on purpose is the play.

Not everything works for every trans person and inside the trans community there is something sometimes referred to as "the coward's they". It's a well known phenomenon where a physical transition gets to a certain point the brain stops easily registering and sorting someone as being their birth sex because they seemlessly look and act as their gender so the automatic neurological system of assigning them a sex value flips fully to the new and desired setting. You see it on conservative media sometimes where they slip up and use the actual correct pronouns and have to correct themselves back over to using the wrong pronouns... Problem being is it causes the same mental redirect issues for a Conservative actively dodging the automatic reaction as learning to use Non-binary pronouns so as a compromise these people use "they" instead because it is easier to trick the sorter and strand themselves in the safe neutral ground where they can identify a person as "not actually a woman/man" without triggering their audience by using correct pronouns for a trans person.

When you use they/them pronouns for a binary trans person it's interpreted by the brain of the trans person as you seeing and reacting to all the aspects of their body that makes them visibly trans and your brain's automatic sex recognition system sorting them into this "not enough" category. It's effectively less hurtful than full misgendering... But it still pings the bit of the brain that is seeing their own body through your perception via your words. It causes they same dysphoric reaction where their mind picks over all the parts of their body that would cause you to react by misgendering or degendering them. The whole point of preferred pronouns is to help us stop that mental reaction from happening as much.

It is perfectly safe to use they/them pronouns for cis people who do not have dysphoric reactions at all and for non-binary people who actively use those pronouns but if someone rocks up looking like they are trying to project a full binary situation it's worth going for the full binary pronoun option because they are specifically putting in the work to be as obvious as possible so that people know that's what they want.