I think this confuses "software is open source" like it's some intrinsic property of the software, and "if you pay me I will sell you the software under an OSS

I think this confuses "software is open source" like it's some intrinsic property of the software, and "if you pay me I will sell you the software under an OSS license which is totally fine. It's a bit of an honor system because is allowed to just publish it for anyone to download for free but it's absolutely something you can do.