I've never encountered this person. Most people who say it, simply say it to save time and effort. I'm working on a product that allows my customer to do X job. I don't need to reinvent a json parsing library or sqlite-like database to get his site online and running his service, and he doesn't care that I didn't write those libraries. I don't feel guilty for skipping starting from zero. If I'm interested in a subject or algorithm then it makes complete sense to reinvent the wheel for my own personal advancement, knowledge, or amusement.