If the job includes initial pilots being based at Cologne I don't see how someone could consider not relocating - at least initially. Perhaps you would have more flexibility after growth when additional domiciles should pop up.
I will tell you why someone should not consider relocating... Because if you join, convinced that it will be temporary, later on, you end up with unhappy pilots who were just naive and filled with illusions.
It happens in all companies so to avoid any disappointment and the bitterness generally seen on forums, one should join with very
realistic expectations. If the job says you have to relocate to Koln then that what it says. Not that later, you will be able to move back home.
At least that's the kind of colleagues I would like to see, upbeat about their new job, location and who will remain as such because they thought about it rather than living in lalaland.