Perhaps it would be less puzzling if the sentence were "Το ορθογώνιο δεν έχει μόνο τρεις γωνίες".
I definitely agree with you, my friend
I agree, but I also believe that it would sound much more natural to say:
"Rectangles don't have [only] three angles."
"three corners" can we include?
A rectangle has more than three corners, this sentence does not make sense.