Don’t be so negative.
According to the Appalachian Trail Conservancy "How does the ATC define thru-hiking?
We define a thru-hike as a hike of the entire A.T. in 12 months or less. These 12 months are not restricted to a calendar year."
If it's good enough for the ATC, it's good enough for me.
I agree that it would be purer to complete each trail in a continuous push, as I noted in the final lines of the piece. The polar expedition community is longer in the tooth and now has a recognised classification system for clarifying such issues (https://pec-s.com). I'm not aware of such a body in thru-hiking.