It's most likely new hires or shuffled from Square enix's other departments, as opposed to XIV's core development team. Could they have borrowed some of the Senior staff for quality assurance? Definitely, it wouldn't really affect FFXIV too much. As senior staff's job is mostly the managing part where they supervise code quality, art quality, 3D art quality, and cross referencing; and procedures.

The real "blue collar workers," you know, the ones actually doing the 3D art and coding wouldn't all of a sudden have 2 jobs. The company would get sued or they'd have to pay them double the salary.
One positive from all of this is that if it's new hires they might be able to keep them after they finish FFXVI and bring them into FFXIV for added operations.

PS. Nice username, I'm gonna sell one of my livers for an RTX 3080 next year.