upstream changed the license in COPYING from gpl2 to BSD3 https://github.com/ferringb/diffball/commit/95c18477dea9c9dd2b362ff40e5a7762195e8d98 but forgot to change the license in all the files the license in the ebuild is BSD3
I think this should be fixed upstream and we pull the new source. Thank you for filing a bug upstream.
Note that LICENSE of the current ebuild was added by ferringb himself: https://gitweb.gentoo.org/repo/gentoo/historical.git/commit/?id=5fd13e1fe5695481b9b3b4f2eefaf84c22b6b1c0 So I think we can assume that the ebuild correctly reflects upstream's intentions. (Of course, it should be clarified in the upstream package.)
fixed with https://github.com/ferringb/diffball/commit/2c7ce43be95fce9430e6724a7d7226c72826bfad see https://github.com/ferringb/diffball/issues/1#issuecomment-864288496
This can be closed; if it wasn't clear it's bsd-3, and the ebuild reflects that.