Yes, that was my reading of "slow" as well, but ironically, GP's point applies under that reading as well: When Microsoft dominated the web, the pace of spec evolution slowed down as well.

Yes, based on that sentence alone, it would be ambiguous. But the author expanded on this with a whole section clarifying what he meant, which is about the rate of feature development and adoption. Not sure if you missed it, but it starts with "I think the Web platform’s most frustrating aspect is also its greatest asset: it’s slow. It’s not just slow, it’s “it took 10 years1 to ship the <main> element which is just a spicy <div>” kind of slow. It’s glacial."

If the author wants to conflate speed of the web with speed of innovation, that’s on them.

Thankfully, the article is longer than that single sentence.

I think that sentence is misleading, and also that the author eventually gets around to making their meaning clear in the later section headed "Slow, like brisket." That section expands on the somewhat opaque initial formulation.