Yes indeed, if a lot of VMs are powered on at once we get “false negatives” (value is divided by larger amount of VMs than is warranted), if a lot of VMs are powered off at once we get “false positives”. (as in your example)
For our specific environment this would not be an issue, we don’t page anybody if such services were to turn red, but nevertheless the value reported would still be wrong, and by multitudes in your example.
I do question however how often this happens really. If I’ve got just 3 or 4 VMs on a host, where shutting down 2 at once would erroneously double the reported values, there is pretty much no risk for CPU oversubscription in the first place (what this check is supposed to monitor and to guard against).
On the other hand, we’re running 50-100 guests on a single ESXi and unless something major broke down we’d never shut down half of them at once (and if so we’ve got bigger issues to worry about than a single occurrence of a check result that displays double the expected value).
Welcome to hear others thoughts on this, for our environment this edge case would not be relevant.