The problem is the 777-300ER wing is already at the max width for code E, sticking a blended winglet on a 300ER will take it to code F (you'd have to cut the span down to fit a blended winglet on thereby wiping out any gains), so to get the wing they need they have to go wider and once out there you might as well have a 10 foot folding wingtip as a folding winglet is pointless.