Devintus and estus devintaj are errors and do not belong in this course. I've made my recommendation to the course authors. It's my understanding that they can't be removed at present due to how Duolingo works under the hood. I've written more about this in other threads, such as this one.
Thanks. Explains why my usual way of thinking about participles doesn't work here (via some kinda broken English).
Esperanto = One who hopes
Mi estas farinta... = I am one who has done... = I have done...
Ili estus devintaj fari tion = They would be ones who had to do that = They would have had to do that
which is totally not the same thing as 'They should have done that'