This was changed (and the message reworded) two years ago in commit 70551c5171abf366b3caa6d22e12892f0da5a95e, after which powertop became capable of reading C-state information exported from sysfs by cpuidle in kernel 2.6.25+ (the data that Jon talks about in this article).
Prior to that, powertop had to read the data itself by making ACPI calls. I suppose this is less likely to work on non-laptops, even when C-states are available, because it's depending on the BIOS vendor doing the right thing (and we know how often *that* happens).
So, upgrade powertop and/or upgrade the kernel?
(You can't count C-state counters in software because when the CPU is in a C state it is not executing instructions. That's the whole point.)