MandM Know what you mean there in your above post. Thanks for adding to the discussion - which is regarding definition and also the particular system(s) being considered.
You are seeing it well. Very well. The modifier in the discussion --- is the 'diminishing' modifier. Once that modifier is applied to the operand or argument (ok ... we'll just leave it at either an operator or a function) -- the result is going to be in the system programming - the rules.
Here, the operation done on a minor third leads to a result - which they define a 'diminished third'. Certainly - we can choose to define the result with that name. And this is where system rules, definition, communications is important (if we choose to make it important that is). If someone chooses to believe that the result is a type of class etc of a 'third' - then they need to explain it based on the rules of the system(s) - in this case, the major and minor scale system(s).
In those systems - the 'thirds' are based on either note spans or based on integer multiples of semitone units between the notes. For most theory that everybody learns - going with the note spans approach is often convenient for discussions, as the number 3 will conveniently pop-up, and easy for many people to understand. Once the term 'diminished third' comes up - that's when people become confused. They just didn't ever realise that the definition 'diminished third' relates to the modifier - or the modification carried out. The words 'diminished third' doesn't at all mean a type of 'third'.
Your last comment is excellent - regarding benefits in having the term 'diminished third'. Fully respecting the music theory - and the incredible work done and accumulated from the past up to now.
You too MandM! --- have a good weekend too. Best regards.