This makes confuse ... you should call it as 5.1.1 (or 5.2) . Now, the 5.1 documentation was wrong, also your press release wrong too. And all articles according to First Release of Proxmox 5.1 were wrong also :(
Then your third 5.1 :( should be 5.1.2 (or 5.3)
1 more ... the Roadmap also wrong :(