Of course, a Mac subnotebook is always possible, theoretically. But with Apple riding an aggressive product release schedule these days, I think a release of a subnotebook is a likelihood. And so does AppleInsider:

This new tiny MacBook, people familiar with the project say, remains in development ahead of its target launch date around the time WWDC rolls around mid-year. It will be both lighter and more compact than any other Mac portable Apple has put forth in recent years, bundling a display of similarly smaller proportions.