I think this is a bug in Curtin. I just tried to deploy a CentOS 8 AMD64 image with an ext2 filesystem and I get the same error. One thing I just realized, MAAS doesn't request that a swap file is created in curtin.cfg. When deploying Ubuntu or CentOS with an ext4 base filesystem swap is not created. It seems Curtin automatically tries to enable Swap in some cases when ext2 is used as the base filesystem.
I think this is a bug in Curtin. I just tried to deploy a CentOS 8 AMD64 image with an ext2 filesystem and I get the same error. One thing I just realized, MAAS doesn't request that a swap file is created in curtin.cfg. When deploying Ubuntu or CentOS with an ext4 base filesystem swap is not created. It seems Curtin automatically tries to enable Swap in some cases when ext2 is used as the base filesystem.