Cartoons and graphics sometimes say more than thousand words.
Today I found the truth table in the cyberspace.


paging icebreaker for immediate application of the MP-RDF. These truths cannot be left unFUDed in public!
That yes/no image is incorrect.
If core is the longest chain and someone is stupid enough to mine an XT block that core doesn't allow, then all XT nodes will follow the XT block fork.
Thus if core continues to be the longest chain (which it will) then XT clients will not follow the longest chain possibly until later.
Not sure if you are being disingenuous or you really don't understand. I'll give you the benefit of the doubt and assume the latter.
BIP101 activates *only* if XT has >75% of hashing power. So once a big block is mined then it doesn't matter if core ignores it, because core doesn't have the hash power to be able to orphan XT blocks by producing a longer chain.
At the point a big block is mined the probability of core being able to produce a longer chain quickly falls to zero. In the first 10 minutes there is a 25% chance, in 20 minutes 6% chance and so on... 1.5%, 0.4%,0.1%. After on hour of XT mining with 75% hashpower there is a 0.02% chance that core has a longer chain. Core is dead in an hour. Thats the real socio-economic incentive, regardless of how many anti-XTers try and claim there isn't one.
If XT doesn't have 75% of hashing power then BIP101 doesn't activate. So both XT and core both continue to mine small blocks and nothing changes.
The chart is quite correct.