In defense of the presence of musketeer and arcanist (and other classes announced at the same time), we will never know what the original plan was for implementation of new classes before Tanaka stepped down from the project. Presence of guilds already in-game indicated that they would be added, but when Yoshi-P took over plans changed and they ended up rehauling all the existing classes before even considering adding new ones.

I'm inclined to believe that this datamining is legit; people have already pulled the entire soundtrack down in .ogg format from these files, and it doesn't surprise me that an unused "JOB UNLOCKED" announcement .dat was found as well. Yoshi-P has said that he wants to bring in as many classic jobs as possible, and the presence of arcanist using grimoires already made this likely. I imagine the addition of scholar (and other staples such as thief, samurai, ninja, red mage, etc.) is a question of "when" rather than "if."