I think you misinterpreted the comment you're responding to; they meant that "sadly, you [the user] never thought to store a high-quality hash (e.g. SHA256) for integrity comparison with your stored data; and the hash you did decide to archive for integrity-comparison was a low-quality one (e.g. MD5) that can be trivially preimage-attacked such that a cloud provider could silently replace your data with a different one — with the same low-quality hash — without your knowledge."

Nothing about rsync.net per se, other than the general idea that any data you put on "somebody else's server" can't be trusted to stay the same if you don't have a high-quality integrity-comparison content-hash of that data kept somewhere.