If that's enough to ruin the theme for you, then I'm pretty sure you should've already been upset by things like Uther summoning Ragnaros and Deathwing or, the Light forfend, Kel'Thuzad. EDIT: Or how about Malfurion summoning Illidan? Thrall summoning Velen? etc. etc. sure they're not all class-specific cards, but it still ruins the thematic consistency as much as having Magni summon the Horde would.
Y'know, the neutral legendaries actually never occurred to me. Oops.
Even so, it's really just the Kor'Kron's lines being a bit silly to me since the card assumes it's being played by Garrosh. For that matter, It'd be kinda amusing to play a Vol'jin against Muradin and have him gloat about being Warchief regardless.
I never really thought the card itself was being played under the assumption that it was being ordered around by the Warchief of the Horde. I just figured that was the audio because that's what a Kor'kron Elite would say.
The characters-as-Heroes thing always struck me as more being about nostalgia and familiarity than any sort of lore or thematic element. That's why they have such a varied mix of old and new characters -- Thrall is the Shaman but not Warchief and Garrosh is the Warrior because of Cataclysm
and everything that followed it, Jaina is the mage rather than Khadgar who realistically would be a better lore fit, Gul'dan is the Warlock, Valeera is the Rogue, and Uther is the Paladin despite them all being largely absent from Warcraft
lore in general for a very long time, and so on.