It works for Bitcoin Core because for Core the wallet backup is the wallet file. For a descriptor wallet you could make a backup of all the descriptor details and you would be able to perfectly recreate the original wallet. But copying descriptors isn't exactly human-friendly and not something that Core offers as a backup solution.

Other HD wallets that don't allow any import of solitary private keys not related to the HD wallet tell you to backup the mnemonic recovery words, optional mnemonic passphrase if that was used too, because that together with the derivation path (if it differs from standard) is all you need to recover your HD wallet.

If you add unrelated private keys to such a HD wallet, you would need to backup the wallet file because the mnemonic recovery words can't recreate the added unrelated private keys and their individual address type derivation.

With file backups there are several issues, in particular if you don't have file backups that represent all your private key pool that may have changed over time if you have added private keys at different points in the past.