What does "Promote this server to domain controller" actually mean, when the server is already a DC?

Your understanding of promoting a domain controller is not wrong. If the AD DS role is installed on the server, and you can point the AD Users and Computers MMC to this particular server, then the DC promotion process is probably complete.

What you are most likely looking at is the checklist within Server Manager that suggests next steps for the server. This may or may not have updated according to what was actually completed already.

How can you verify? Try the following commands and verify that your server in question shows up in the list of domain controllers:

PowerShell: Get-ADDomainController -Filter * | Select-Object HostName

or NETDOM: netdom.exe query DC

Also, run these two commands to make sure that replication is functioning successfully for the name of your domain controller:

repadmin.exe /showrepl and repadmin.exe /replsummary

You should not see any errors or failures, and hopefully your server in question shows up in the list of successful replication partners for AD.