You cannot add All Authenticated Users in FBA site. You can do it in Windows sites only. There is however a work around as suggested in this link by creating a role in the directory service, add all existing users to it.
I wonder whether the link provided by you will be of any help. But here is the Powershell equivalent of that code:
$Url = "http://sitename"
$GroupName = "group name"
$PermissionLevel = "Read"
$Description = "group description"
$web = Get-SPWeb -Identity $Url
if ($web.SiteGroups[$GroupName] -ne $null)
Write-Host "Group $GroupName already exists!" -foregroundcolor Red
$web.SiteGroups.Add($GroupName, $web.Site.Owner, $web.Site.Owner, $Description)
$group = $web.SiteGroups[$GroupName]
$roleAssignment = new-object Microsoft.SharePoint.SPRoleAssignment($group)
$roleDefinition = $web.Site.RootWeb.RoleDefinitions[$PermissionLevel]
Write-Host "Group $GroupName created successfully" -foregroundcolor Green
$user = "c:0(.s|true"
$web.SiteGroups[$GroupName].AddUser($user, "", "","")