If you read the full article, it covers the law in California which has three tests. Two of those tests are straightforward (company time and company equipment), but the third (related to the company's business) is less straightforward. My impression as a non-laywer is that the courts will probably take a somewhat narrow view of that third test, but the actual language in the law is pretty vague and some companies are involved in many fields.

EDIT: May be a bit less narrow than I thought. This [1] journal article cites some relevant case law.

[1] https://digital.law.washington.edu/dspace-law/bitstream/hand...