All more reason to use a passphrase over the system being proposed here. In the event of being subjected to a $5 wrench attack, then you can reveal your seed phrase to the attacker, and they can steal the small amount of coins that you have stored in the base wallet. However, the majority of your coins which you have stored in additional secret wallets which are hidden behind one or more passphrases remain not only secure, but completely hidden; the attacker has no idea they even exist.
As long as attaker has no proof that a given address belongs to you. Similar situation would be with having different coins at the same seed - it would be obvious to find BTC and ETH (+
clones), but having for example Monero (or any other exotic coin) at address generated from the same seed is different story.