In this case, "arbitrarily large" contradicts the text that follows,
which describes the circumstances where that might not be true.
I've carefully chosen the words of the title, and it doesn't
contradict what follows, as far as I understand. It says "Emacs is
now capable of editing files with arbitrarily long lines", in which
"capable" means that it can do it, but will not always do it. The
circumstances that are described in the text that follows tell the
reader that the remaining cases in which Emacs would choke on such
files are outside of Emacs' responsibility, they are the
responsibility of major and minor mode writers.
I see that you've already reverted. Sigh.