As per BODATSA (following the latest revision) Hilliardiella oligocephala is recognised as a synonym of H. elaeagnoides. POWO to update.
Swelankomo, N.; Manning, J.C.; Magee, A.R. (2016). 'The genus Hilliardiella (Asteraceae: Vernonieae) in southern Africa'. South African Journal of Botany. Available at: https://doi.org/10.1016/j.sajb.2016.05.010

For discussion, see: https://www.inaturalist.org/flags/530851 & https://www.inaturalist.org/observations/38184313