Look up the meaning of Loophole. The Flashback was before the events of Birth By Sleep, so it in not in the events just a minor flashback.
And also Samhain, he said that BBS was 9 to 10 years before BBS. And are you acquainted with the world of fiction where anything can happen.
It doesn't change the fact that Pure Blood heartless existed, minor flashback or not, loophole has no meaning in this.