That's slightly misleading. The applications that get directed to a "screen-wibble" terminfo entry will behave as if they are running under screen, because the

That's slightly misleading. The applications that get directed to a "screen-wibble" terminfo entry will behave as if they are running under screen, because there will be no terminfo ritm/sitm capabilities and they won't emit any control sequences for specifying italics. tmux's own behaviour does change, but that is secondary, and a bit of a bodge. (The applications will emit what they think is something else, but will end up emitting the ECMA-48 code for italics, so tmux refuses to recognize it.)