How was I illegal?

I was living in London - not in the USA. And I was perfectly legal in London as I had residency because it was part of the European Union and I'm also a European citizen (German).

I only arrived in the USA after I won the Green Card.

Please get your facts straight.

The person who wrote the article is leaving things out. Nobody is allowed to stay that long in the States on Work Visas. Nor are they permitted to go from job to job. That's what you do when you are illegal.