True, DDD pretty much introduced the idea of anything is possible now. I think it felt like that because i watched the back cover movie like a month ago and didn't feel Luxu had that kind of personality when interacting. Also, the beginning of fragmentary passage showed Luxu watching the ending of BBS, even though Braig was there doing his thing during the final battle. I guess he had to go throw on his coat and grab the box real quick lol.
Actually looking back on it now, that could have been him watching the end of the original Keyblade War
also Luxu was a student of the MoM and MoM must of had a huge impact on him. Plus he has lived for many, many years and has changed over time. During back cover, he was a very young man and the youngest of the apprentices it sounded like. Plus taking on different bodies comes with many different masks he had to wear over the years