Read about that trick on a woodworking forum last year. They called it a wash coat and said it blocks the soft grain from soaking up too much. Makes total sense why it works so well.
Seriously, my buddy had the worst blotchy stain job on a pine table last month. Told him exactly what @paiger71 said about a wash coat, and it saved the whole project.
My cousin tried a wash coat with shellac on pine and it dried too fast. Had to sand it back and use a different sealer. Honestly, the type of wash coat matters more than people think.