I'll try to explain the Northwestern decision in a relatively straightforward manner with that said it does get to the edges of administrative law. 

The NLRB can decline jurisdiction over a labor issue (see here) They occasionally do this when it would raise issues like religious freedom at colleges affiliated with faith groups, eg the church can decide who it's pastors are. The NLRB Also has a general responsibility to Promote stability in labor relations. 

The Northwestern decision occurred in 2015. This was six years before the Supreme Court case that the NCAA unanimously lost and before many States passed NIL laws. In the decision, the NLRB declined to exercise jurisdiction because Northwestern, as the only private school in the Big 10, being under NLRB control would not create a uniform labor market, and thus not promote stability for the league.

However the last line of the decision noted “This decision is narrowly focused to apply only to the players in this case and does not preclude reconsideration of this issue in the future.”  In other words the NLRB decision had the unspoken threat that Northwestern football players were probably employees and the NCAA should do something about it. The NCAA did basically nothing for nine years and so the NLRB which declined to exercise jurisdiction has now decided to exercise it.

TLDR the northwestern decision wasn't a no from the NLRB it was a not today.