Where Warren says "America’s big tech companies have achieved their level of dominance in part based on two strategies: using Mergers to Limit Competition,..." he argues that they achieved dominance by making good products and were already dominant when they started making major mergers, and that Warren doesn't understand this.
It sounds like he's missing the whole point here: google, amazon, and apple of course did achieve dominance by making a good product, but then they leveraged that dominance over one product to dominance over the whole tech sector with a combination of more good products and anticompetitive moves. Warren does not argue that companies that make good products shouldn't be allowed to get big - just that they should be scrutinized and not allowed to make anticompetitive moves. Indeed, all of the tech companies she has proposed breaking up would still be huge companies with "dominating" products (including control over their respective platforms - search, app store, amazon store); but hopefully the ecosystem on those platforms can flourish without competition from the platform companies.