It just makes brute forcing meaningless, since it'd be quicker if you brute forced the address instead of the seed phrase.
Brute forcing is already meaningless because the entropy size is big enough to make it impossible. Adding a very small step on top of it can not be considered a security measure. Also if we assume that the seed phrase is leaked, it is safe to assume that the extra words are also leaked.
Also saying one impossible thing is easier than another impossible thing is like saying going to Uranus is easier than going to Neptune! Or in this case not even Neptune but only one of Uranus's close by satellites.