I agree! It's been my experience that people genuinely believe they can reliably tell real empathy from fake.

It may be possible that the detection heuristics a given individual relies upon might, upon occasion, be a bit less reliable than could be hoped for. I've witnessed both false positives and false negatives.

Again, you're right. People do earnestly and honestly believe in their ability to detect genuineness.

I wasn't even the person you responded to, and I still felt the warm fuzzies.

The book emphasizes this fact. If you don't actually care about the person you're trying to influence, if you aren't acting in what you believe is their best interest, they can tell (usually). We have marvelous words in English for this, like "smarmy" and "skeezy".