Yeah, it feels wrong that dpkg gets file system code, gets code for one particular file system.
I disagree. If there is a significant optimization that dpkg can implement that is only available for btrfs, and if enough people use btrfs that there would be significant communal benefit in that optimization being implemented, and if it is easiest to implement the optimization within dpkg as seems to be the case here (indeed, it may only be possible to implement the optimization within dpkg), then it is perfectly reasonable to implement the optimization in dpkg. Dpkg is a low-level OS-level utility, it is entirely reasonable for it to have OS-level optimizations implemented within it.
jik