I'm not sure I understand.
Do you mean composers who were not proficient in any instrument?
That's how the other respondents seem to understand it.
I know that Hector Berlioz couldn't play the piano.
Or do you mean composers who could write a manuscript without trying anything out on the keyboard?
Composers who are addicted to the keyboard are frowned on by other composers.
Bach called such composers "clavier knights," and I call them "keyboard junkies."
In this case, Mozart reigns supreme.
Mozart could even write out all the instrumental parts without writing the score first.
Of course, we must also consider Beethoven, who is often portrayed as a keyboard junky before he went deaf, but wrote his Ninth Symphony, among other masterpieces, after he went deaf.
I'm not clear on whether or not Puccini was a keyboard junkie.
When he was composing Madama Butterfly, the first motor vehicles came out.
He bought one for himself and he was fascinated.
He drove a little too fast and had an accident.
So he could do nothing all day except sit in an easy chair with his foot propped up.
Since he couldn't sit at the piano, this delayed his work on the opera.
However, I also read somewhere that Puccini wrote the entire last act of La Boheme at a table where his friends were playing cards.
I can't imagine how both stories could be true, because La Boheme was earlier than Madama Butterfly.
Among all the famous composers, the most notorious keyboard junkie is the Nineteenth Century Russian composer Cesar Cui.
Cui didn't have a formal education in music theory, so he had to sit down at the keyboard and fish.
It took him ten years to compose an opera which a well-trained composer could write in a fraction of the time.