This also means you can use any random garbage as a seed input for BIP39 wallets - you can, and someone already has, used stuff like "hello" as the seed phrase, and it works fine. Though most wallets will probably complain and/or disallow you to use something that doesn't follow the standard wordlist and checksum format.
Yes you can actually use just any random words you like to create a seed phrase but most of those wallets that uses BIP39 standard will not be able to recover that wallet, as for the checksum it can easily be solved and then pick the word that matches it. But it is bad idea to actually manually generate your own seed phrase because it doesn’t create the randomness that your wallet or Machine will pick