I feel like 3 seconds to load a page is still slow. I assume that is time to build a page that doesn't hit cache, and that Wikipedia is using something like varnish to cache pages most of the time.
Still, 3 seconds to load a page feels like a slow page and should be a lot faster.
Did you even read the article, or just skip to the comments when you saw the graph? He spends a whole section talking about Wikipedia's aggressive caching strategy, and how 96-98% of the pages are served from Squid cache servers.
Also you didn't even scroll down to the next graph, where he shows the average page load time (for logged in, not anonymous, users) as 800 ms. (which is obviously larger then the average page load time served from cache). The first graph was the average amount of time it takes to save a wikipedia edit.
3 seconds is page saving time for editors. Since wikipedia is a read-heavy site, I would expect that to take longer than loading a page, as writes will always be more expensive and less optimised.
By comparison, uncached page load time appears to be ~800ms
I would be surprised if the servers/instances responsible for rebuilding old pages had any other responsibilities. If that's the case, DoS-ing by flooding old page versions would likely only bring down that particular site feature.
Still, 3 seconds to load a page feels like a slow page and should be a lot faster.