It is definitely a bootable disc. It was my main boot drive until I installed Catalina on another drive.Only reason why it would not show the HFS+ disc in that case would be that the disc is not bootable.
When I am in Catalina, system preferences recognized the El Capitan disc as a start up disc ans I can select it. So the OS sees it as bootable. But when I restart it always restarts to the Catalina drive.
Basically I can’t seem to boot back onto my El Capitan drive since installing opencore. It doesn’t “stick” when using startup disc and it isn’t “visible“ in the opencore bootpicker.