Why August 1999? Seems quite specific?

RedHat's IPO. Seemed like as good a date as any to anchor "when Linux went mainstream". I'm sure there are other dates that are arguably at least as good here.

Ah right, that roughly tracks. Just wasn't obvious where "August 1999" came from.

Kind of amazing timeline in hindsight by the way; Linux was "only" 8 years old at that point.