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.
Yes, I agree. But any block greater than 1Mb would be rejected until XT had 75% hash-power support... a fork is only meaningful to people if there is some appreciable percentage of hash power behind it.