Could be other reasons for the difference in TCO.
I'm assuming IBM, much like all other large corporations, uses a standardized (customized across the entire organization) OS image on the machines. Those OS installations come with bundled software (say, encryption, monitoring, license analyzers, etc). Tools to help and automate things.
Now, it could just as well be that the tools themselves are shitty.
There's a paradox here. Windows OS can't really tightly integrate tools because everyone would yell "Monopoly!", so Microsoft simply doesn't include lots of tools that come preinstalled on Macs. And if one or more of these third party tools are deployed for the whole organization, as a mandatory installation, those tools might generate a lot of cost. Is this end users' fault? Unlikely. It just might be that whatever encryption or AV tool some big shot suit picked just sucks. It crashes often, corrupts data, loses saved credentials, desyncs with the policy server, etc. So this generates cost, and Microsoft / Windows / Users are blamed.
While I don't dispute the claims, I am sure the TCO reasons require some digging.