That's actually quite unprecedented I think.
My children are both half-Basque through their mother, and because they had not grown up in Pais Vasco, they wouldn't be eligible to play for Athletic even through their mother. It wasn't enough alone to be an immediate blood relation of a person born and raised in Pais Vasco.
I admire their club for their strict eligibility policy and the fact they have never been out of the top division in their existence, even spawning Atlético originally as a reserve team. Amazing they're still competitive in today's global market.
Then you have Laporte who only had Basque grandparents and Amorebieta who was born in Venezuela.