A recent promotion cites a long-standing mystery of Icelanders having unusual robust health and longevity. It claimed the explanation lies with soil enrichment from volcanic ash that contains sulfur and selenium. While that explanation might contribute, is the major reason for the longevity of Iceland’s citizens their relative isolation and not being heavily exposed to chronic disease-causing viruses?

Your intuition is spot-on here, that this is yet another example of a rare group of remote humans experiencing robust health and well-being, and a long-lived survival in spite of being in harsh environments seemingly constituting an ongoing threat, if anything, and not inherently a benefit to survival and longevity. In the case of Iceland, the extreme harsh cold much of the year constitutes a huge negative most people would choose to avoid if they could. So the fact such natives are thriving, indeed, cries out for an explanation and you have the true answer—they are dodging a bullet in the sense of only occasionally getting exposure to viruses brought in by visitors, or their own travel to other countries where they might become exposed to a chronic disease-causing virus and bring it back with them. Given that the nature of such viruses is that of being a low-level presence, making them hard to find and provide evidence of their negative influence, also means they will not be transmitted easily from person to person. So it is more likely that someone will ingest a beverage contaminated deliberately with virus during a trip, in providing an effective vector to get a new infection going. Then, that individual may not even spread it to others locally, but would suffer the consequences personally, and be little influence on health statistics nationally.