A fill has a twofold purpose, It is a transition from one musical phrase to another and it is a drummers opportunity to make a statement on the music. It is not an opening for the drummer to make a musical fool of his or herself.
Good fills are based on one thing alone. The drummers knowledge of the music being played or to coin a phrase "context fills". Ringo Starr was particularly good at this, so were many of the drummers for James Brown and Motown.
The idea of filling through the groove is not new. It's just not emphasized. Playing fills in context is more a matter of playing experience than solo practicing. The idea of a musical fill is a combination of what you play but where you play it.
The word fill implies space in the music where you connect one musical idea with another. The end of a phrase is usually the spot where you can insert a fill, there are other gaps in the music where you can fill as well. Don't play a fill if there is no hole. Filling in the wrong spot will disrupt the groove and the flow of the music.
I am not advocating "less is more or more is more", what I am advocating is hearing and listening is more. If you can "hear" the music then you will play a fill connected to that sound. It's really that simple.
The groove continues