Board Thread:Lore Discussion/@comment-66.243.250.142-20150427025156/@comment-2804:14D:1C76:80EB:E0A8:4428:C026:374B-20181219152344

The wiki itself has a trivia about it:

To address the confusion that arose from the existence of a Dragonborn predating Alessia, Michael Kirkbride, former writer at Bethesda, stated, "Alessia didn't have the power to absorb dragon souls. Hers was a much more nuanced power: to dream of liberty and give it a name and on her deathbed make Covenant with the Aka-Tusk.

[//elderscrolls.wikia.com/wiki/Miraak http://elderscrolls.wikia.com/wiki/Miraak]