SouthPark I hear you, or at least I know where you are coming from. You are pointing out that the below (a diminished third) has only 2 steps, so why would it be referred to as a type of third interval.
I think the others know that the diminished third has only 2 steps, but I think they want to point out that there is value/reason why a 2-step interval is sometimes shown in sheet music in diminished third notation, and then referred to as a "third" (diminished).
Some of my background is in programming, so here is a (not quite fitting, but the best that I can do for now) example:
The mathematical term (3-1) = ? You say why would it not be 2 ? It's clearly a 2, no ? Let's not call it 3, or anything else.
However, there may be benefits in calling the result still a "3" but with a modifier added. Maybe call the modifier "minus" where it is understood that minus refers to "-1". What could be the benefit ? Maybe keeping the term a "3" simplifies programming, variable labeling, relative offset, or something to that effect.
So, that is why I do not mind so much that a 2-step interval, in certain situations, is being referred to and shown as diminished third. I trust that there are benefits in music why this methodology was kept. Wish you a good weekend.