Actually, I think the way the title is phrased lends itself to misinterpretation one way or the other. If you follow the link from the Forbes article to the SA article it draws from, the actual body therein starts off saying "Google Pixel is now the fastest-growing major smartphone brand in the United States." It's equally easy to infer that they are saying that, of all major smartphone brands, the Pixel is the fastest growing among them, within the US market.
In either event, the key takeaway from this is that the article recognizes Google as being in that company, as a "major" smartphone brand.
As to its being the fastest growing, being so new its growth year-over-year is likely to be relatively greater than established brands. You're unlikely to see fast food chains like McDonalds, Subways, Starbucks earn a similar mantle whenever a relative newcomer enters the mix as the bar for relative growth is so low for them.