I always interpreted that as a case of unreliable narrator. Of course the Seanchan Empress will claim to have a direct lineage from Hawkwing, but everything we know about Seanchan royalty indicates that it probably isn't so. They assassinate each other all the time, and nobles and even commoners can be raised to the Blood.
IDK, there seem to be limits to how far they will go against the Imperial Family, and the organization and record-keeping practices of the Seanchan strongly suggest they'd be keeping careful track of that. You'd think there'd be at least a hint in the in-canon commentary if their belief was untrue, as we see in the BWB concerning the claim of Mayene's royal family. That whole "operating on mistaken beliefs" is a major theme of WoT. In this case, I think the trope RJ was aiming at was the idea of a hero king's bloodline being something good or awesome or the rightful ruler. I think it's more valuable to RJ's themes and ideas if the Seanchan rulers do have the ancestry they claim. If they don't, he really blew the chance to make use of that fact.
“Tolerance is the virtue of the man without convictions.” GK Chesteron
Inde muagdhe Aes Sedai misain ye!
Deus Vult!
*MySmiley*
