I think it's very safe to assume that no major US based platform has 'real' E2E encryption. They're almost certainly all a part of PRISM by now, and it'd contradict their obligations to enable government surveillance. So the only thing that's different is not lying about it. Though I expect the other platforms are, like when denying they were part of PRISM, telling half truths and just being intentionally misleading. 'We provide complete E2E encryption [using deterministically generated keys which can be recreated on demand].'

Signal is open source