I understand it the opposite way. It is a 4.6 inch display on the front when used as a normal phone, which then unfolds to be a tablet.
I am perfectly content with a 4 inch iPhone SE so I see no problem with that form size. I'd assume that I'd use it half the time as a folded phone, then when I wanted a bigger display, unfold it.
I find the large iPhones impractical. Too big to practically use one handed, too small to be a good tablet. I see it compromised both ways. I could see in maybe 4-5 years that a folding phone would be far more practical.
And this is the point. People have different use cases, different needs and requirements. This is a very early example of something that could expand to be far more useful. I've said in this thread time and time again, nobody is suggesting this is ready for the mainstream. It is for people who have money to throw away to test things. If that isn't you, then don't do it.
And the same question could be asked about the price of the iPhone XS max. Is it worth the combined price of a MacBook Pro + iPad 9.7?