it is an excellent couple, sounds wrong to my American English ears.
Better to say, they are an excellent couple.
Unless the sentence intends something inanimate, such as it is an excellent pair of shoes or what have you
Yes you are right when " couple " means two people who are married or who have a romantic or sexual relationship, but " couple " also means two people or things that are together, and in this case we can say "It is an excellent couple."