It mentions "AWS S3 or compatible service". The S3 API is a de facto standard for object storage services, and there are numerous implementations of it.
(Including many which you can easily self-host like Minio, for those who are following along at home and weren't sure whether that was just limited to other cloud services)
As you mentioned, huge fan of Backblaze B2. No affiliation, just a satisfied customer. Can also pair it with Cloudflare for even less expensive traffic serving.
