The implication here is that the risk of in-flight incapacitation can be managed by limiting flight time. Would this be a correct assumption?
No.
The rule is per annum. Regardless of whether you are flying, mowing the lawn or just loafing in front of the TV.
A spot of typing into your preferred search engine will bring up a series of articles and papers on the subject. But the wikipedia one is as good as any.
wiki
For some more enlightening reading I recommend Sally Evans' papers on the subject of Pilot Incapacitation. Here is one on pubmed for a starter...
The annual incapacitation rate of commercial pilots