Yes, I think it's wrong, or at least way over-exaggerated.

You can run a grid to supply approximately 80% renewables (long-term average) without significant technical changes.

Only if you want to get the last 20% to renewables, you get technical challenges, e.g. related to synchronization and load-matching. But that is also not unsolvable problems, e.g. instead of relying on the inertia of steam turbines you can "just" build specific-purpose fly-wheels to do the same thing. It's just less elegant.

Source: Volker Quaschning "Understanding Renewable Energy Systems", too lazy right now to look up the exact page.

This is also consistent with the section they quoted. Generally, the load matching in grids is done by the system itself. If you add more wind and solar, which depends on the weather and location, you have to more large-scale intervention, e.g. allow generation re-dispatch. But that doesn't immediately imply that this is a dangerous process.