And if a critical effect generates more damage, nowhere it says it doesn't count as part of the original attack.
Nowhere does it say it does count, either.
The rules says so. Critical hits (and effects) from the attack are resolved immediately, under the very own "dealing damage step" of the very same attack, which makes all damage part of the same attack.
What you are trying to imply, however, is that during an attack there's 2 different sources of damage, the attacker, and the critical hits, as if the latter were a different entity than the attacker, or were not part of the same attack, or more precisely, the same 'dealing damage step', since you will be dealing damage two times, once for the 'attack', and another one for the 'critical hits that generate damage'.
Edited by Jehan Menasis