I am not sure of the meaning of your colleague’s sentence; depending on his opinion in fact.
In my opinion, in the sentence “Diamétralement opposés ? Rien que ça ?“, I think this means “That much ?”
Maybe your colleague thinks they are not that much “Diamétralement opposés“.

OR

This can be a way to say that they are more than “Diamétralement opposés“, then this would mean “Only ?” ; a way to underline what you have just said.

The sense is the same in your affirmative and in your interrogative examples: "rien que ça" is used in a ironical way, to point out an overstatement. The interrogative form emphasises the doubt about your choice of words "diamétralement opposés", in quite the same way that an exclamative sentence would.

{Person 1} Mon fils vient de s’acheter une Porsche.

{Person 2} Une Porsche, rien que ça !

and

{Person 1} Mon fils vient de s’acheter une Porsche.

{Person 2} Une Porsche, rien que ça ?

carry the same meaning. You can use the interrogative or exclamative form to express the surprise.

I guess the difference is that you can also use the interrogative form with a non-ironical meaning, for example with a genuine surprise:

{Person 1} Je vais prendre une petite salade.

{Person 2} Une petite salade ? Rien que ça ?

In this case, you could also use the expression "C’est tout ?". Recognizing the meaning would rely on the context. I think your colleague was ironic, unless you really were understating it with "diamétralement opposés"?