The bolded I can understand. What I felt Monday & Tuesday was definitely not out of the ordinary. Wednesday was only slight worse, and I thought about testing on Thursday, but didn't have a valid test.
But, relying on a test to determine "Day 1" doesn't make any logical sense. The virus isn't going to care when you test. When using things like "After Day 5... " and such, if someone waits a day or two to test, that messes up the "what should happen", at least in my mind.
"From onset of symptoms" makes more sense to me, in which case I'm now on Day 8. Feeling better than Friday & Saturday (so far) but still cough and congestion. No fever, but never really had one. So, still contagious? (Rhetorical question).