You're arguing when you don't understand the basic proposition. First off, he's not "purchasing a product from Windows specifically for group policy"---that is part of the OS. Second, his primary point seems to be total cost of ownership rather whether or not certain functionality is available.
He's saying those things are more expensive to implement on Linux---either you have to buy them or pay more in labor to get them. He's not wrong.
From your own examples, OpenLDAP takes considerably more time and effort to setup.
MS Active Directory is one command, five minutes of installation, and a reboot. The defaults work---as in, nothing else to configure manually---it even opens the necessary ports in the Windows firewall. It includes the group policy functionality he indicated, and it works out of the box with every version of Windows anyone has any business running anymore. Yes, the OS license costs money, but intelligent deployment really makes this a minor per-server expense (i.e., buying Datacenter licensing with decent virtualization density).
Nagios and SCOM both cost money---it's either licensing fees for the packaged version of Nagios or labor for the source/DIY version. Puppet costs money to do for Linux what Group Policy does for Windows. The labor to sustain the Linux solutions will probably cost more even if it is as simple as SCOM/GP because MS has a huge pool of labor to support their product. I can probably find dozens of competent AD admins within a reasonable commute distance---the number of competent OpenLDAP, Nagios, and/or Puppet admins is going to be significantly lower.
Microsoft is actually very good at catering to small businesses and enterprises---this is where known costs, straightforward deployment, quick and effective configuration management, and simple sustainment are important.