In the context of this blog post, it's a group of people (or individuals) who work to improve the developer experience, usually including testing, code quality work, and practices. It sometimes also includes tooling, release work, frameworks, etc.
I think the original question was more "How do we measure that we are increasing engineering productivity, what constitutes engineering productivity?".
Yes, but the post is about a group of people or an individual who works in a particular field, not the subject of productivity itself. So I defined the phrase as used in the post.
One way to define that is in terms of the customer: quality (no defects), calendar time to deliver, benefit to customer, and value (cost-benefit ratio). So you can say things like, "if we refactor this code now, we can schedule less time on maintaining it in the future, spend less unscheduled time fixing bugs in the future, and deliver features in less time with less effort."