Yes, there ARE enamel primers.
When I started painting real cars, cheap shops were still using synthetic enamel topcoats, and some of the primers were enamel.
Enamel primers are typically NON-sanding, because of their drying characteristics.
I've had all kinds of problems trying to topcoat a urethane repair when some doofus had previously shot hardened acrylic enamel or acrylic urethane over a synthetic enamel primer.
You can get away with it once, sometimes, but when the repair topcoat hits it, the previous topcoat wrinkles like you shot paint stripper on it.