For example, PEAQ is a project being developed on the IOTA Tangle infrastructure. Is this considered within the bounds of on topic?
At this stage, I would try to be as inclusive as possible.
It's not like we are going to create a string of new sites for each project using this technology. Unless someone wants to make a compelling case that this is an entirely different domain or audience, or that it will actively alienate the core community here, I would open up your scope to be inclusive of these subjects.
If it causes problems down the road, you can always disallow them later; but I find it better to err on the side of inclusivity, and try not to anticipate too many problems before they become prevalent in actual practice.