New Fedora 33 installations did get Btrfs by default for automatic partitioning, but did not get compression. Enabling compression was a Fedora 34 feature, so only new Fedora 34 installations get compression enabled by default.
But you can add it to fstab. Strictly speaking you only need to add compress=zstd:1 mount option to the / mount point. It is a file system wide mount option. As @dalto mentions, it only applies to new writes from the time the option is enabled. Since old writes have already happened, it’s completely OK to just enable compression from this point forward, you don’t need to defragment with a compress option.