10/8 is a class A range (high order bit = 0), but CIDR says that you can separately describe which bits are for the network. Unless this interview was conducted more than 20 years ago I'd say your interviewer was mistaken.
Is that what you meant to say? The interviewer (me) thought that 10.0/16 was fine, which I think agrees with your analysis. The interviewee said only 10/8 was correct.
(Re-reading my GP comment I realize that my role might have been ambiguous, so I've clarified it.)
If you said 10/24, I'd immediately think 10.0.0.0/24. 10/8 would certainly be valid. 10.0/16 is too.
I remember when CIDR came around back in the 90's. Man people were pissed off when you would call out a 'Class A/B/C/D/E' block. I prefer to always be specific when writing my CIDR. I understand when people don't do it.
Be liberal in what you accept and conservative in what you send.
