Well doh... seed phrase is supposed to be secure in the first place. It's essentially the private key to all your private keys, or whatever addresses correspond to that seed.
Keep that stainless steel bolt and nut safe.
12-word seed phrases do seem secure unless (a) there are flaws in the implementation that reduce entropy such as the pseudorandom number generator, (b) attacker knows something about your phrase or (c) there is a quadratic improvement in computing power such as with quantum computers.
Interesting point, bitcoin's private keys are 256-bit - was Satoshi just wasting space?