A decade ago JavaScript developers had little or no trouble working cross browser, writing small applications, and churning out results that work reasonably well very quickly. It isn't that cross browser compatibility had been solved, far from it, but that you simply worked to the problem directly and this was part of regular testing.
That older generation did not have the benefit of helpful abstractions like jQuery or React. They had to know what the APIs were and they worked to them directly. The biggest problem with this is that there weren't many people who could do this work well. Then shortly after the helpful abstractions appeared and suddenly there was an explosion of competent enough developers, but many of these good enough developers did not and cannot work without their favorite abstractions. These abstractions impose a performance penalty, increase product size, impose additional maintenance concerns, and complicate requirements.
The ability to work below the abstractions is quickly becoming lost knowledge. Many commercial websites load slower now than they did 20 years ago despite radical increases in connection speeds. To the point of the video this loss of knowledge is not static and results in degrading quality over time that is acceptable to later generations of developers who don't know the proper questions to ask.