That was before he became the Winter Knight by an entirely new deal.
Being the Winter Knight isn't paying off the old debt, he swore himself to lifetime service in exchange for power.
And yet, what Dina said is true. Mab said that when three favors are fulfilled, he will be free of faerie influence. That was at their first meeting in Harry's office. When on the battlefield in SK, Mab offered the deal of being the WKnight and erasing all favors owed. When he didn't take the deal, it was taken off the table.
Yes, Mab doesn't have to ask for favors anymore with Harry being her Knight. But, if say he unknowingly does her a favor, it would free him of any faerie influence, as she said back in SK, and the WKnight mantle is a faerie influence. So her rule was set first, and her being fae and the Queen, it should supersede Harry's vow.