Estinien does contemplate that for a moment longer, if only because Geralt looked so shocked by what he'd said. He feels like it'd be a mistake to try to compare the events too directly, when he doesn't know the cause or true extent of the one suffered by Geralt's realm. It sounds like it may have been worse, or at least more chaotic. The most recent calamity had fucked things up, for sure, but it was something society had been able to move on from in a relatively short period of time, no matter what scars were left behind.

"...I take it the... collision of your... spheres... it wrought significant change upon your world?" Even more weird terminology to get used to, great. "Were the other spheres very different from your own?"

If that can even be defined - does Geralt know which sphere his people were from originally? Maybe that's the difference. While the other shards were distinct culturally, they mostly included the same ingredients as the Source, and were being added bit by bit. While things changed, it was more like weather - not fundamental changes to the makeup of reality.