In this specific instance I wasn't joking or wrong.
It's the same reason Arteta is ineligible for England. FIFA require players to have the nationality of the nation they want to switch to at the time they played in their first 'official game' (not a friendly) for their original nation (at any level - even youth).
For Schneiderlin to be eligible for England, he would have needed a British passport in 2004 - when he first played for France in an official competition (he played in the qualifying round of the UEFA U17 Championship).
The Diego Costa situation is different, he played in two friendlies for Brazil (not a competition), friendlies mean nothing in terms of tying players down to a nation. Had Costa played in the Olympics - even for one second, he wouldn't have been able to change allegiance to Spain.