On average I get the first signs of activity in 12-24 hours, which is about right I think.
If fermentation was to kick off in 4 hours though, I'd assume I over-pitched my yeast and the cell density was higher than the 1.5B estimate.
I agree with the 1st part of your quote in terms of a normal ferment starting within 12-24 hours.
I respectfully disagree with the 2nd part though.
If an adequate starter is made and pitched at high krausen, it is not impossible to see fermentation with 6 hours.
I have had it happen on 3 of my last 5 batches and within 12 hours on the other 2.
I have used Woodlands 1.5B per mL recommendation and Brewers Friend calculator.
So if I am overpitching, then both of my main resources are way off.