I would transalte : He probable does not understand. What is the difference between "he" and "they"...?
You mean in the Czech sentence? No difference. If you want to be 100% clear that it is plural you can use rozumějí but rozumí is correct as well (the sázet paradigm of the 3rd class verbs).
The word bank offered me "They probably don't understand my question," which I chose and was accepted, but with a typo. Reported.
The action this suggests is that we manually add these contractions everywhere because Duolingo's handling of them is buggy. I do not see that happening unless Duolingo gets out of handling contractions entirely.