Trello is hugely popular amongst software developers, but I think the point being made is that it's also popular outside that niche. At work it came in through the back door being used for software planning, but quickly became the preferred tool for managing any sort of project. There's a similar story with me showing it to my wife, who just finished managing her PhD in history with it.
I'm a data scientist and I've been using it as an informal Kanban board because JIRA is way too complicated-looking for me want to even try to learn. Trello is about as dead-simple as it can get. There's already too much in my brain.
I know a lot of smaller dev teams use it for lowercase-a agile projects and I currently use it for all my personal projects. It's basically the antithesis of heavyweight tools like Jira.
