Yes he was. Warlocks in Warcraft lore are mages, shamans that practiced arts of summoning demons from the twisting nether. There are very few characters in Warcraft lore that were trained as Warlocks from the let go. Even Gul'dan, the most well known Warlock was a Shaman. Kael wasn't your typical Fel Magic Warlock like orcs became, but he was still aligned with the Legion and summoned them. Abilities don't define Warlocks per se, it's more of allegiance with the Legion and using them to achieve your pesonal goals that make you one.