It seems like with APFS, the backups DO NOT work. It is unable to create the folder on the Time Capsule disk. Using the 2TB Apple Airport Time Capsule personally, since upgrading to 10.13, unable to backup anything. Luckily the iCloud backup/sync is still working, so at least I have that.
I would assume Apple is going to release an update for the Airport/Time Capsule (hopefully) to make it compatible, or update the file system on the Airport/Time Capsule to APFS, which would be even better!
For now, I would just get a cheap external drive and backup there, since you can customize the disk format to be APFS on external disks.