However, it's worth noting that extending the seed with a passphrase is not going to add much security if it's not too complex or if it's stored with the seed in the same place.
Yeah, that is true, if you set a BIP39 passphrase, it must be one that is difficult to bruteforce, it isn't recommended to set a passphrase that is easy for you to remember or memorize, as some newbies might do. It should be a strong passphrase and then backed up in a different location from the seed phrase, that's when it can be useful in protecting ones funds.