The Austrian Group left Qualiflyer (and became a partner of LH-miles&more) as per march-26-2000.

Flights after march-26 with Austrian Group Airlines (Austrian, Lauda, Tyrolian) don't qualify for Qualifyer-miles anymore.

But in your case it seems that when ordering your tickets, or at check-in (or both), your Delta-frequent-flier number was on file with your ticket-agent and/or the airline (how else would they have found out about your Delta-membership?).