Furthermore, Bitcoin Core will only fork away from Bitcoin XT when XT has 75% of the hash power behind it.
The very second a >1Mb block is created by XT, we'll have two different chains. That's a fork in my book.
We'll have 2 different chains how?
Let's imagine the scenario:
%5 of the miners are mining on XT, and %95 of it mines on Core.
XT miners finds a block and adds it into their chain (let's say it's chain A)
Core miners reject that block and finds another one (let's say it's chain B).
How will chain A proceed? Core miners will never give a damn f* about chain A, so There's only 5% of the network mines for it (imagine the difficulty).
Chain B will proceed because it still has 95% of the mining power.
Anyway, if majority of the HR doesn't switch into XT, it'll be fine for Core & Core miners.