I suppose the confusion happens between perfect relative pitch and perfect pitch. Perfect relative pitch is when you can tell if a guitar is in tune or sing a harmony note that sounds right.

Perfect pitch is when somebody says "I usually start this song on Bb. Would someone play me a Bb please?" and someone (possibly daylia) sings you Bb, as close as their voice is capable of.

It's a memory thing. Some people learn what what notes named a certain thing and always played at a certain pitch sound like, and it stays in their heads. They don't have to work to remember - it's just there. It's fascinating to me to think about what makes some people able to learn what a G sounds like and remember that sound for their whole lives while most of us need a tuner or known-to-be-in-tune instrument to find a G.