Yea, many specialists can focus on damage, Nazeebo, Sylvanas, Sgt. Hammer also could be assassins with your definition. For me specialists are characters with great waveclear and push dmg. Junkrats attacks are aoe so he has great waveclear and has long range attacks which are great to push. For me he is more mobile version of Sgt. Hammer. Splash dmg, long range attacks, skill to escape, skill to root/displace.