I never disputed that; Valve said that the neurotoxin incident in the Aperture Science labs supposedly takes place in 1998, so yes, Portal 1 would occur some time after that. And then that same article you've linked suggests the most specific possible date range is the first decade of the 2000s, and Half-Life 2 supposedly takes place 20 years after the Black Mesa Incident.
Edit: Nvm, the neurotoxin incident takes place a few days before the Black Mesa Incident. But still, the time range still checks out.
Edit #2: By the way, it's very clear from the Elysian Fields-style ending of Portal 2 that no one on the Valve Portal team has ever been to Michigan.