For almost a year now, we’ve been doing something that’s considered quite risky for an open source project: Paying contributors.
Le gasp! You mean, you'd consider sharing some of the money you get from selling a product with the people who contribute to it? How revolutionary! Next you'll be offering them paid vacation days!
What's really funny is that this isn't even what they're doing. They got an OSS funding grant, which gave them extra cashflow, which they then chose to spend on freelance work. That they chose to target people already working on Discourse only makes sense, but they're not awarding bounties for PRs or anything like that — they find a piece of work they don't/can't do, pick someone from their contributor pool that they think can do it, and offer them a (negotiable) contract.