The gist of it has to do solely with billing rate and hours.
Sure, but it's not "what rate should I set?". That's generally off-topic as per the help center and we usually point those people to "How can I determine a reasonable salary to ask for?"
It's even tagged "freelancing".
Of course, that's what it's about. We also have legal and software-industry tags.
I see nothing here that has to do with "navigating the workplace".
Contract negotiations and communication with clients about expectations or business agreements have traditionally been considered on-topic here under the "Finding and maintaining employment" topics mentioned in the help center. We do tend to avoid going too specific into the details of freelancing arrangements. I would probably redirect a question like "What is the best system for billing my clients?" to Freelancing even if it could perhaps be considered on-topic.
I still don't understand why this is on-topic...
Because it didn't get a single close vote. That's a pretty strong indicator that the community considered it on-topic.
...and wasn't moved.
While I can't speak for the rest of the team, I'd never migrate a question that wasn't closed or that I wouldn't feel comfortable mod-closing.
Would it have been answered if an independent plumber was concerned about billing a client? A landscaper? An actor?
I want to say "sure, why not", but it's true that this kind of contract work doesn't come up here often and I would expect to see close votes because it's outside most users' area of expertise. But at the core if the question is "I agreed to do X for Y$ and the client is now offering me only Y/2", it doesn't really matter what X is.
Would most of the questions on freelancing.stackexchange.com be considered on-topic here as well?
Probably. Looking at their current top 15, I'd say around 6 or 7 would be considered on-topic here. Another 4 are technically on-topic but they're the sort of question where I'd refer the OP to Freelancing.