Yeah, that's the thing that kinda sticks out to me as well. Being able to prove your noble lineage is the primary concern of anybody with a connection to royalty or a hereditary title. If someone is anywhere in line to inherit the throne of a kingdom, then everybody in that kingdom knows it. And anyone who's anyone in any of the neighboring kingdoms is going to know as well. The guy would have to be incredibly out of touch to not know who that woman was. Plus, if the baroness is looking to get married to this guy's son, the first thing she'd do is provide her lineage to prove what a good match she'd be for his son.
It'd be one thing if the guy had already promised his son to someone else and was now honor bound to go through with it regardless of a better match turning up, but there's no way the guy doesn't thoroughly check the lineage of someone looking to arrange a marriage to his son.
I've had a thought of 1 way that The Earl may not know that his would-be daughter in law will 1 day be a queen
The reason that he may not know is because while she is the heir to the throne on her fathers side on her mothers side she's only(?) recently inherited the title of Senior Baroness that comes with rulership of A Large Barony
That means that the most likely answer is that somehow the earl knows that she's recently inherited the heir to the title of Senior Baroness that comes with rulership of A Large Barony from her mother but doesn't know that she will 1 day inherit the throne from her father
(In fairness, I remember taking a certain ... postgraduate course ... some time ago, and I remember that a large portion of the class had their minds blown at the concept that heirs were not established until after death. I think that the confusion continues to exist today in popular understanding.)