Although (brain wallet produced by human) --> seed key
is recognized and accepted as a bad idea,
f(brain wallet produced by human) --> seed key
where f is a easily remembered math procedure such as modulo(x), may form an acceptable key
[...]
WTF does it even mean that something 'is recognized and accepted as a bad idea'?
Sounds to me like an argument brought by someone who has no arguments
I think what Spendulus is referring to is that brainwallets that are derived from human generated passphrases tend to get bruteforced sooner or later. At least those that are purely based on words that can be found in dictionaries and quotes that stem from books and song lyrics.
There are many examples of brainwallets that got swiped by attackers, however it is of course impossible tell what percentage of brainwallets is affected since the total number of brainwallets is unknown. Nonetheless the general recommendation on these forums and other social platforms has been to stay clear of brainwallets (whether justified or not).