As far as I know, there are some leading theories that many autoimmune diseases are linked to a part of our immune system which exists to hunt parasites, and goes somewhat haywire in people who never encounter parasites their whole lives, which is common in many European countries and in North America.
If this is the case, then the benefit you get from the worm is not in any way "symbiotic", it is purely related to your own immune system overreacting to the absence of any worms.
Regardless of the reasons in a few specific cases, the vast majority of people lead much better lives without gut worms than those that do have gut worms. So I don't think there is any reason to doubt that gut worms are parasites by the definition I gave, even if they might have a beneficial effect in a few specific cases.
If this is the case, then the benefit you get from the worm is not in any way "symbiotic", it is purely related to your own immune system overreacting to the absence of any worms.
Regardless of the reasons in a few specific cases, the vast majority of people lead much better lives without gut worms than those that do have gut worms. So I don't think there is any reason to doubt that gut worms are parasites by the definition I gave, even if they might have a beneficial effect in a few specific cases.