Yes its absolutely true. Sure most got a away with horizontal mounting, but many do not.
Couple problems; 1) If the seal leaks, the oil will start to seep/drip. Coil will soon burn out from internal Hot spots. 2) Even if no leak, any internal air will be along the top, possibly allowing the same hot-spot condition, leading to possible failure.

An epoxy filled coil (the "Vibration-proof ones) is the way around this if you want horizontal mount.