First, S3 and a SQL database aren't comparable. But I think you're bringing up S3 because they're using Litestream to ship WAL frames to S3. Go read the Litestream documentation; Litestream syncs to basically anything. They don't need to "migrate to another S3 compatible backend"; they can migrate to almost anything that can save a file.

It's a super confusing argument regardless, because the industry is lousy with "S3-compatible backends".