They aren't. A run-on, 70 word sentence for example wasn't easier to store in your head, or not require the occasional second read-through back then . Readers then still had to go through all of that. This "it's just outdated" idea is employed primarily to hand-wave poor literacy performance and reading requirements.
Gravity's Rainbow came out in 1973 and has a lot of the same difficulty, but people don't call it 'outdated' or treat it like a foreign language. These are just books you have to read slower than a news article, and that's alright, but there's a fine line between needing more time and not being able to get through it. That study showed it conclusively.
