libsdl can draw with directfb and directfb is a common USE-flag used in many ebuilds. But the libsdl ebuild always disables directfb-support. The directfb support should be accessable with the "directfb" USE-flag Reproducible: Always
Your reasoning is kind of flawed. Yet, in this bugzilla the closest pick seems to be bug 503130 - while it's about libsdl2, chances are the reason was the same.
directfb is actually only used in 14 packages and some of those are obsolete. Support for directfb is generally being dropped across the tree. I notice the upstream website has gone away. It's intentionally turned off and unsupported in libsdl because of lack of multilib support in DirectFB. The directfb use flag was removed at the time multilib support was added to libsdl.