There are for-profit companies that hire devs to work on open source projects?

Tons of them, yes. Most big companies you know even - Google, Facebook, Amazon, Apple, RedHat/IBM, Valve, Intel, AMD, even NVidia.

Are they earning money with something else or how do they make money from this?

Some of them do support contracts (RedHat), some make money with data server services (Amazon, Google), some sell hardware and the software is drivers, or an incentive for people to buy more hardware (Intel, AMD, NVidia).

In Tech Paladin’s case, Valve uses KDE software on their Steam Deck, so they want it well maintained, features they like added and bugs they care about fixed. To make that happen, you have to either hire people yourself, or pay another company to take care of it. Valve does the latter with a lot of the open source projects they use.