Complex sentence: With LVM, a logical volume is a separate block device, while a Btrfs subvolume is not a separate block device and it cannot be treated or used that way.
Split into simpler sentences: With LVM, a logical volume is a block device in its own right--while this is not the case with Btrfs. A Btrfs subvolume is not a separate block device, and it cannot be treated or used that way.