a brain wallet is where you choose the words.. and most of the time brain wallet users choose between 1-6 common words that are part of a known phrase..

a seed wallet is where 12-20 RANDOM and UNCOMMON words are used.

the article stated
that is a 0.0000018% success rate.

now although there are 171,000 words in the dictionary. its estimated that only 3500 words are used commonly.

so imagine the password is 1 common word.
thats a 1 in 3500 chance of a hit.

so imagine the password is 2 common words.
thats a 1 in 12,250,000 chance of a hit.(3500 x 3500)

so imagine the password is 3 common words.
thats a 1 in 42,875,000,000 chance of a hit.(12,250,000 x 3500)

som brute forcers know that even in the 3500 common words, some are not used, so they could get the odds down. they also know that when using more than 3 words its more likely that a sentance structure was used (phrase or quote) so they know what words naturally follow grammatical structure and what words dont naturally follow each other in a sentance.

so although the odds of having 12 common words can be upto:
1 in 3379220508056640000000000000000000000000000 chance.
brute forcers can reduce that down to:
1 in 1000000000000000000000000000000000000 chance.
just by employing some grammatical rules to cut down on the variations possible.

which is still extreme for 12 word sentence.. but. its highly important to not use sentances/quotes that follow grammatical rules. it is also important to not use the 3500 common words. that way 12 random non common words can be:
1 in 3138428376721000000000000000000000000000000000000000000000000 chance.

so in short a brain wallet of 3 common words is:
1 in 42875000000 chance

so a seed of 12 random and uncommon words is:
1 in 3138428376721000000000000000000000000000000000000000000000000 chance.