It's absolutely true. Some fluxes are very strong, and will cause visible blue/green corrosion on copper within hours. Others may take days or weeks. When you
see copper pipe that is all gnarley with green/blue corrosion on it, especially around the joints, it usually means some lazy plumber neglected to clean off the
excess flux, as he is required to do by at least some plumbing codes, and certainly by the dictates of Good Practice.