Application could be very basic (calendar, address book, calculator, alarm, etc.) - by default. Or users could make their own simple apps - nothing fancy, Arduino like cooperative multitasking, simple screen drawing, maybe a few widget helpers.
I can imagine this being a fun thing to use & play with. :)
[0] https://www.amazon.com/Nokia-8110-TA-1067-2-45-inches-Unlock...
Still, being able to write your own apps for it would be nice. Even if very low level APIs & you would have to handle distribution yourself if you wanted to share them would be fine. But I don't see any information about that after quickly checking their page.
BTW, a simple mobile terminal with cellular connection that you can program could be preatty useful in comparison to hacking a regular Android phone to only do what you need could be useful as well. For stuff like single use online terminals to give to field crews.
Of course (as I wrote here three years ago¹), a non-programmable dumb/feature phone without updateability would also be adequate for my personal needs for what I need a phone for.
1. https://news.ycombinator.com/item?id=15358342#15366260