Jesus, what a bullshit. It's not about screen size, it's about reading experience. If you think that longer lines making something easier to read - try to read non-trivial book with 100 characters width.
Books tend to use proportional fonts which can tolerate greater average line lengths (measured in in characters) than monospaced fonts of similar size (same em), and in any case won't have a constant length in characters line-to-line, but instead vary based on the specific test to be similar in total width.