If a companion has no taunt slotted, then they are not taunting - they are drawing threat passively. Companions seem to draw aggro quite a lot, to the point where you may need a taunt to get the mobs off of them (or just kill them).

https://forums.elderscrollsonline.com/en/discussion/576585/companion-aggro-is-off

https://forums.elderscrollsonline.com/en-gb/discussion/610915/zos-why-is-companion-aggro-not-fixed

So far ZOS has not seemed to change this, which may mean it’s intentional - as one of the early posts in the first thread suggests, having companions draw a lot of threat means less attention on players who need companions to help them through combat, which is their primary reason for existing in the first place.