Refactor (not refractor :P) basically means restructuring the code . So changing bits here and there of the codes so that 1, it works again and 2 (not always though), works better/is more efficient.
In that case I think they needed refactors probably because they (Epic Games) changed the way the engine operates the code in the new engine version, so certain features were no longer working properly with the current code.
I’m no expert, but that’s probably something along those lines.
"Macklin Guy" commented "Unreal Engine 4.19 Upgrade Status
We’re still working hard on the migration of Tower Unite from Unreal Engine 4.18 to 4.19. We are currently hampered by a last minute bug which is preventing us from releasing the 4.19 upgrade. The bug, which affects workshop items in Condos, has proven to be quite difficult and requires a rewrite of the code. We’re working as fast as we can to resolve this issue so that we can release the 4.19 upgrade to the public."