He was not transphobic in any way. You've just been incredibly childish, congrats.

Albeit I don't really like referring to myself as trans, I will for the purpose of this. The guy made a valid point, and you as a fellow trans person should understand that it IS a possibility. The moment we begin our transition, and even before that, we know how hard and how important it is to legally change our name and our documents in general; simply because of the implications it may cause if it they do not correspond with what you are and how you present yourself, it happens in our own countries, and it will happen in other places too, specially in countries that aren't openly LGBTQ-friendly like Korea.

If you want to go to there, it's a beautiful place, and you're going to enjoy your trip. You won't get attacked, and obviously not everyone will be prejudiced against you, as with anywhere in the world, there's the 50/50 chance of someone being or not being prejudiced. If I were to suggest something, personally I wouldn't openly say that I'm trans and expect major acceptance. Either only say to people you know, or keep it to yourself and just be a nice and kind person.