The crease on the One Plus Open is visible in the unboxing vids and reviews I have watched and looks like an uneven ridge. It is not as pronounced as some of the Samsung phones, but is there. Can you really see Apple offering an iPhone with a large folding OLED panel and an exterior OLED panel for of a 16 Pro Max or £1399 which is already expensive?
Personally I think Apple would spin its marketing to suggest we are getting an iPhone and iPad in one, so assume by adding an extra £600+ on top of current iPhone prices, would be a justification. Hopefully they can see by the foldable reaction from other manufacturers, there is a market for them, but most people don't want them.