It mentions “a source told the Seattle Times”, but…they didn’t. The Seattle Times was reporting on a purported whistleblower posting to a public forum (what you linked).

I think there are two sources.

The fuselage panel that blew off an Alaska Airlines jet earlier this month was removed for repair then reinstalled improperly by Boeing mechanics on the Renton final assembly line, a person familiar with the details of the work told The Seattle Times.
...
Last week, a different person — an anonymous whistleblower who appears to have access to Boeing’s manufacturing records of the work done assembling the specific Alaska Airlines jet that suffered the blowout — on an aviation website separately provided many additional details about how the door plug came to be removed and then mis-installed.