Storm Brand of Indecision says "... and those around it" despite being single target.

Storm Brand inherently deals area damage, the transfigured gem does not change that. It sends beams to some number of targets, including the branded enemy), and deals damage in an AoE around those targets hit by beams.

Storm Brand of Indecision only hits one enemy (the branded enemy, if it has one) with a beam, but that beam still causes damage to that enemy and those around it, just like the base skill's beams.