When Tarlyra – a Humakt initiate – goes into combat, she wields some of the power of Humakt, standing in for the god in the mundane world. That’s what Rune Magic is, after all. She also likely calls on some spirits to aid her in minor ways – protection, demoralize, bladesharp, etc. She might get spells cast on her by her friends and companions as well, and maybe even by her temple.
When Anaxos the Horali goes into combat, he is aided by friendly spirits to aid him in minor ways as well. He might also belong to a martial cult (if permitted by the local talars). Anaxos is Seshnegi, and his talars have enrolled a local (and limited) version of Humakt – giving him access to Truesword. Although he has good armor and well trained, he has less direct access to magic than his barbarian equivalent.
But if the talars demand it, Anaxos is supported by the magic of their wizards, which if properly prepared is every bit as formidable as what Tarlyra has, and maybe even more! Long term spells such as dampen damage, boon of Kargan Tor, or warding against weapons make Anaxos a terror on the battlefield. But only if Anaxos has the support of his talars and they command their zzaburi to cast those spells.
That’s how sorcery ends up working in Malkioni societies.
Spirit magic exists throughout Glorantha, because spirits exist throughout Glorantha, so ordinary people everywhere make use of spirits – unless they have a good reason not to (as with sorcerers and dragonewts past their first stage). According to some scribes, it was actually dragonewts that first used spirit magic, although they give it up in order to spiritually progress beyond the Crested Dragonewt stage.
How would a duel between Tarlyra and Anaxos would go? That really depends on what sorcery spells Anaxos can get cast on him by the zzaburi – and on how many Rune Points Tarlyra has!
Not that sorcery is very socially useful. It just is not as useful for free-wheeling adventurers. Or members of societies that don’t want to support a bunch of specialist sorcerers.