Payment by royalties is pretty standard in publishing. Twice a year off previous half-year's sales is fairly common in non-gaming publishing for instance, though I don't know the details for Onyx Path. Lag between submitting a draft, time for editing and printing, and six months after release date can be pretty long. If the publisher then doesn't bother to cut a check, what's your recourse? If you're very unlucky you might already be into them for two or three books.
For the record, WW and then OP never paid by royalties to freelancers, or at least not in their gaming sourcebooks. There may have been one or two occasions for somebody, somewhere, but as long as I worked for them, me and everybody I knew got paid half upon delivery of a final draft and the other half upon release or something similar. You also always got a kill fee if for some reason you produced your work and it wasn't released. There was one time when I got paid 100% upon printing, when somebody dropped out of a product at the last minute and I had to produce my whole portion over a weekend, but again, the first thing that happened is that I got sent a contract with the terms laid out.
OP and WW never expected anybody to work for free, and anybody saying that is not telling the truth. From day one, I was told never accept work as a freelancer for any company where you didn't get a contract, because the contract spells out a) how much you're getting paid, b) what you're getting paid for, and c) how you get paid if for some reason they don't use your stuff. Also, things like comp copies and the like. During the various work I did for Exalted over the years, there was only one time where developers expected me to work without a contract, didn't give me one when I asked, and when OP was apprised of it they immediately rectified it after reaching out to me through two other developers and then an employee of the company proper. I was informed in no uncertain terms that was not how they did business, and that matches my previous experiences when WW was in business.
Contracts and pay are a big deal because they protect both the freelancer and the company.