If they have been on social media during the last 10 years it would be pretty much impossible for them to not know that tranny is now considered a slur when referring to a trans person. Maybe an old fart who just watches cable TV could be unaware, but the odds of being on social media and not being aware is pretty much zero.
I also thought maybe it was a misunderstanding, but really it has been long enough for people who are online regularly and have a blog.