Second Age section comments here!
Main thread: https://basicroleplaying.org/topic/6602-guide-to-glorantha-group-read-week-7/
Deep Discussion: https://basicroleplaying.org/topic/6603-guide-to-glorantha-group-read-week-7-deep-discussion/
Here's another of one Lawrence Keogh's WIP, this time with Jeff's edits:
The Gift Carriers were another name for the Knowledge Assassins (mentioned in the Umathela writeup). They really only targeted the God Learners there and God Learners elsewhere had their own dooms.
As for it being a Deus ex Machina, when the Jrusteli overstepped the bounds of magic is one of the two instances of when Arachne Solara intervened (the other was the Sunstop). However it's not made clear where she intervened and my guess is probably the fall of the Machine City (in which the Gods did become active) rather than Umathela.
IMO the Gift Carriers are not super-powerful entities. What they did have was a knowledge that allowed them to kill God Learners. This knowledge wasn't made by the Gods to punish the God Learners (although many people will think that it was). Rather the knowledge was unintentionally created by the God Learners themselves and that the Knowledge Assassins took advantage of this.