Hi Southpark
Another way to think of it (in my simplistic way) is it's equivalent to double sharps and double flats. Why do we call any note a double sharp/flat? F## is actually a G. Ebb is actually a D etc etc. But of course we do have double sharps and double flats to make sense when writing music sometimes.
It's a similar logic with diminished 3rds.
Diretonic is a very knowledgeable chap, and you only have to look it up on-line to find out that he's right.
Of course you call it whatever you like, but it is a diminished 3rd.
The joy of music theory
Cheers
Simon
All round average Jazz, Blues & Rock player.
Currently working towards ABRSM grade 8.