Yes, you're right. My impression of the Romanian Duo course is that it hasn't been proof-read or checked by a native English speaker.
A bit like "double u, double u, double u" in place of "world wide web". Nine syllables to abbreviate three!
"În o" and "în un" usually get contracted. "Într-", comes from the latin "intro", as "în" itself.
Funny thing to have a longer word in the contracted form, isn't it? :)
But it helps with the rhythm: "într-o" and "într-un" flow nicely, "în o" sounds awful.