It's only showing American startup activity as a proportion of global startup activity. I might have missed it in a quick read, but nowhere did I see total numbers by nation, or by startup city.
Let's shift perspective slightly. Is American investment in startups decreasing? Not compared to China or whatever, but compared to itself? Or is it staying the same, or growing? Looking at Pitchbook data, the biggest year for US venture capital was 2015 ($82.2B), with 2017 close behind at $81.9B, and 2018 looking to be about on par. So stable, basically.
What this tells us is not that the US startup activity is in any way unhealthy, but rather that the rest of the world is figuring out how to do it, too. And what's wrong with that? How is that a negative? We don't get that from the article, because it's stuck in this misleading mode of lost competitiveness from is comparison model.
This is why everyone should read Factfulness. No other book I've ever read has helped tune my bullshit detector so well.