Deal with and pre-empt cross-browser compatibility issues, ensure accessibility, iterate on the UX, deal with stateful-but-not-persisted applications, optimise for constrained devices whose hardware you do not control... And keep up with whatever technologies exist to help with those challenges.
The article even mentions some of these. Yes, you could just say back-enders should know about all that, but... There's a limit to how far a single human can vertically integrate their knowledge. For example, the same argument could be used to claim that 'developers' should be able to handle all the challenges of the back-end, of database adminstration, of operations, and why not, let's throw in business development, design and content strategy as well.
In the end, it mostly depends on what you can afford. If you're a small, bootstrapped startup, you have a couple of people doing everything. But as you grow, specialisation can become more efficient.