I fail to see how. I know a thing or two about this under the hood technology and the leap was made when they did the farm = personal phase, garrisons reused what the farm did. They lagged like hell at the start, because every phase takes resources and since garrisons were bigger than farm, these resources were bigger as well, so with lots of people in garrisons, servers were just running out of resources - that's all. The tiny bit of "innovation" that they added / enabled later - which was the queue - was completely trivial.
What you say was "quite a feat" wasn't a feat at all (and the mess was completely predictable, they just failed to test because they were in a rush).
PS: Really, there's nothing interesting in garrisons in terms of technology. I might be forgetting something, but...