Well, yeah, except maybe not. If you're Apple or Google, you're trying to get customers to buy a product that you've built yourself, that you maintain yourself, and over which you essentially have total control. (Google was originally fiddling with some fairly open concepts, but lately they've been making moves to enhance their control over Android.)

In such an environment, you've gotta sell this product as the best thing since sliced bread, because your customers sure as heck ain't gonna have any ability to modify it themselves.

Jolla, on the other hand, is offering a product that gives you a much finer degree of control. Sure, it doesn't come complete with all the bells and whistles of iOS or Android; but then, you yourself can make up the difference with your own software.

Maybe we now live in a world where only one or two companies can write their own software, and everyone else has to just live with the results. But if not, then Jolla is acting rationally -- the point is not to show that they have a full-blown iOS / Android level OS that already serves every need of the end-user, but rather that they've got a customizable system available that has already done all of the hard GUI work for you. You only need to install your own middleware into the system, and poof, your mobile device is ready to ship.

Anyway, that's my theory.

tl;dr: Jolla doesn't need to show that they themselves have produced the next iPhone. It's enough to show that they've got a pleasing and usable GUI, that licensees can enhance to turn into their own custom iPhone.