AC10 is what you have naked. Try removing the clothes from Astarion and you will see that his AC remains at 10. Remove the armour from, e.g. Lae'zel and you will see her AC drop to 10 (assuming she is not wearing anything else which bumps AC).

The quote you posted tells you this:
"While you are not wearing any armor, your armor class equals 10 . . ."