This seems like the most important point. Is it normal for research groups to take on PhD candidates who they are unable to support in the field of interest?

{Takes on Quantum Computing grad student} + {has insufficient Quantum Computing expertise in group to even advise} seems... pretty bad.

PS: And being from a tenured family and having grown up exposed to academia, I'm inclined to take author at their judgement here, given that taking on students in hot topics meshes with how the incentives of grant funding, publishing, and prestige work.