I've never understood this claim but I've seen it quite a bit

like, maybe this could be an extremely specific attack on an individual known for some phrase, but otherwise, you're not doing a successful dictionary attack on a long password just because it's a sentence in more or less proper english

even if you claim there's some loss of randomness due to the patterns of letters and spaces -- which is true -- that's more than offset by how easy it is to remember and type out an *extremely long password*

"and so mary said, bring me a cup of orange juice with my toast"

good fucking luck pal