Apple's total sales in 1978 [1] were only 30% higher than Vector's in 1979 [2]. Yeah, the industry growth at the time means comparing even consecutive years gets dicey, but I don't think the gap between the two was enormous at that point. Comparable to Apple in the late 70s sounds pretty major to me.
Also, it is reasonable to say "major" is an absolute description that just means "pretty big" not "one of the biggest". As you mention, their sales peak (IMO, past the company's relevance peak) is pretty big in absolute terms.
Extrapolating this disagreement into "much of the article may be false" is ... confusing.
[1] https://guides.loc.gov/this-month-in-business-history/april/...
[2] http://www.s100computers.com/Hardware%20Folder/Vector%20Grap...