This may be true, except to the best of my knowledge as you point out, DOSBox is 'GPLV2+', i.e. GPLV2 with the "or (at your option) any later version." clause left intact.
However, as a whole, DOSBox is licensed (currently) GPLV2, so any xBR/xBRZ stuff which is licensed either GPLV3 or LGPLV3 is incompatible with the current distribution of DOSBox, unless DOSBox changes its entire license to GPLV3.
(any xBR/xBRZ stuff which is licensed GPLV2 or LGPLV2.1 is fine, though)
Due to the fact that certain distributors (Steam, (and MAYBE HumbleBundle?)) use DRM/TiVo-ization and distribute certain games with DOSbox packages (with sources included), changing the license to GPLV3 might hurt the project significantly.
"When life gives you zombies... *CHA-CHIK* ...you make zombie-ade!"