As the writer described the meaning of 猫の額ほど and how to use it, the following sentence came after it.
My friend told me that this phrase means something like, "If you say 猫の額 when the owner of the house understands that saying and heard you, you'll be considered impolite (so becareful).".
Make sense, since cat can't understand Japanese. But then I wonder why it isn't 猫が分かる人に instead. Maybe cats here are a metaphor or representation of people, which would support my friend's claim.
It could also mean that using the saying in front of a cat is rude since not all cats have 狭い額, but... It's a cat, who cares? Unless the writer is trying to emphasize something.
Either way, I want to know how and why.