I don't doubt that that many watts is easy to cover - eventually. The problem is that it can be instantly turned on and off, whereas the grid takes time to shed load or add capacity.
I found a figure on Wikipedia saying that the NL's 4.7GW worth of offshore wind capacity is 16% of their total electricity demand nationwide. 4.7/.16 = 30GW total, so this theorized computer load attack would represent about 10% of their grid's total capacity. Can their grid add and shed that much load that quickly? That's the part I doubt.
You skipped over the part where I point out that my assumptions are completely off. These numbers presume that all computers in all Dutch households are hacked, running and connected to the internet. 5% of that would be on the high side even.
So a more realistic "attack" would be able to move demand, 0.5% of the total grid capacity. Switching on/off one smelter in an aluminium factory, is probably more than that. Hacking a major charging-station company and switching off their chargers is probably more than that even.
I understand the direction you think, and I agree that the combined power usage of "consumer devices" is big. But the larger power system is rather well protected by an attack on these devices through the diversity of these devices and the diversity of their setup (consumer firewalls, routers, individual protection, in-house fuses, local load killswitches etc).
The solar devises lacks this diversity, as the article mentions. There are few brands, and all of a brand need to connect to the one cloud service in the exact same way. So this does have a single point of attack. Whereas "switching on/off all personal computers in a country" is of an entirely different level.
According to World Population Review and corroborated by other sources, the Netherlands has 91 PCs per 100 people. So your assumptions aren't that far off, actually. If anything, your assumption is lower than reality because a household is defined as 2 or more people.
I found a figure on Wikipedia saying that the NL's 4.7GW worth of offshore wind capacity is 16% of their total electricity demand nationwide. 4.7/.16 = 30GW total, so this theorized computer load attack would represent about 10% of their grid's total capacity. Can their grid add and shed that much load that quickly? That's the part I doubt.