It was common knowledge. Much of the history of this time was destroyed or lost. But this was all known at the time, and it was also known who descended from where. The hybrids are described explicitly in some texts like Enoch, and with features like long necks, very tall stature, a glowing aura or eyes, and even more reptilian features (descended from Seraphim angels), it would’ve been easy to identify them. There’s even some evidence the hybrids like satyrs and cyclops are related to these ...