Looks like they were trained in nasal breathing and asked to close their mouths during the nasal breathing session.

"Unexpected activity appeared in the superior and inferior frontal gyri independent of working memory" - seems their brain was focused on properly executing these nasal breathing instructions/training - which would be expected.

Doesn't say too much considering sample size.

A better study could be configured by observing twins (one with broken nose at one point, leading to majorly deviated septum and inherently limiting nasal breathing vs the other twin)