"These ships are royal" would be "Kesra/keja lōgra dārōñar issi" (from 'kesa', 'kesys', 'keson’, ‘kesor’). Which is the sentence that Xenon- is suggesting in the comment above.
But the 'kesi' in this Duolingo sentence is the plural form of 'kesy' and essentially means "these” when referring to inanimate, countable things. (See here: https://wiki.dothraki.org/High_Valyrian_Vocabulary#kesy and here https://wiki.dothraki.org/High_Valyrian_Pronouns#Demonstrative_and_interrogative_pronouns)
Compare a similar matter and David J. Peterson’s (username Dedalvs) answer here: https://forum.duolingo.com/comment/27873577