Not entirely correct—historically all we had were serifs (after handwriting & blackletter/fraktur) until the end of the 19th century. No one made the statement “serifs are more readable” because what alternative was there? When Caslon’s sans-serif became a thing, and Akzidenz, Gill Sans, Futura, and the rest spread and sans serifs proliferated, there was natural aversion to these “bastardized” letters. Serifs were preferred merely for familiarity.
So any statements about readability may have only come about during the middle/late 20th century. Studies near the end of the 20th century (this one by NASA being the most referenced) found sans-serifs either more readable than serifs, or no significant difference. We still use them in books because we just always have. It’s not broken, and it’s familiar.
Anyways, this statement isn’t incorrect, but in a historical context there’s only a relatively small window of time in which these statements were made, and I think the sentiment “serifs are more readable” is dying out in our sans-serif-heavy digital age.