It's at least plausible. A decade before Haida Gwaii was first seen by European explorers there was the 1763 Siege of Fort Pitt. During the siege the British famously attempted to infect the natives with smallpox by giving them blankets that had been used by smallpox patients. Whether it worked or not is a subject of debate, but the Europeans clearly had the idea that they could intentionally spread a disease to harm a population.

I don’t know where yall are from but my schooling in Canada taught this as fact. I don’t recall specifics since it was so long ago so I have nothing to back that claim up.

I was unaware that this was not a known thing. Indigenous Americans were treated abysmally by the people who invaded and stole their land.

I won't count myself as an expert, so feel free to skip this post if it isn't helpful. My background is in U.S. history. I can't speak to Canadian or British documentation.

Oral history and written history are two different matters and people will weigh them differently. I have not read Mr. Swankey's work, but I would want to know more about what kind of "vetting" the Haida did before I can judge the impartiality of the work. (Edit to add, if you want to talk about the issues with oral history, my family's oral history describes tribal members removing blankets from the graves of people who had passed from illness. This would be post-civil war northern plains. When I worked in collaboration with Crow staff from the Little bighorn site, I was informed that blankets were a status symbol and all us women had to wear them for a special event. I remember this because it was 90F out and I was ready to strangle the chief of interpretation with said blanket. But if this is true, then it could be interpolated that blankets--in this area--were valuable enough both practically and socially that some people would have enough motivation to take them in that manner. However, an entire narrative can not be written solely from such stories. Rather they should be small pieces of the bigger picture, supported or refuted by multiple sources.)

According to a U.S. history professor I worked with who was trying to trace primary sources for this aspect of history on the U.S. side, he believed this claim originated from a passing mention in a letter written by a British officer (I'm sorry, it was so long ago I can't remember the officer's name and date of the letter, and I don't want to dox my coworker since his professional opinion may be unpopular).

He had been unable to find any further follow up in the written record, specifically in relation to any official U.S. government policy or general implementation of such a strategy. This was some years ago so I am unaware if his research produced any more primary sources. Thus far the letter he read has been the only direct primary source I've seen in it's entirety. Like you, I have come up short when trying to locate documentation. The Wikipedia (yes, I know) page for smallpox lists several sources under the "biological warfare" section related to what is written there and it could be interesting to look into those publications.

I also have had questions about how people understood disease at this time, and how would they determine the unlucky guy who would get to "gift" infected goods and potentially be infected himself. I have to wonder how this was supposed to work logistically based on their (lack of) understanding of viruses. Maybe I'm over complicating this, but I would not be eager to mess around with anything from a smallpox victim any more than necessary.

This is a difficult subject to examine though and find the actual facts, both because of how long ago this was, and because of the emotional aspect. I still remember a (different) professor I had in college when I was studying for my bachelor's in history who was giddy about how many white people had died from tobacco and how they deserved it and it was great the native tribes got revenge like that. I had a hard time accepting him as a (reasonably) unbiased source afterwards, though he would count as an expert.

In 1796, the first vaccine was discovered and it was for smallpox. ~30 years after this stuff happened. It's quite interesting if you'd like to do a quick read.

https://en.m.wikipedia.org/wiki/Smallpox_vaccine