The F7 is a secondary dominant for Eb. It gives the feeling that it's about to do an F7 -> Bb7 -> Eb motion, but after giving you those feelings the plan is cancelled and it goes to IV of Eb i.e. Abmaj7 instead. "Gotcha! You though we were going to F7 - Bb or something, didn't you!"
In order for something to work as a dominant or secondary dominant for something, the dominant-tonic motion doesn't actually have to happen. If you press pause at the F7, you can analyze its meaning in the context, before knowing what's going to happen next. If your mind anticipates something, the anticipation is true and valid at that moment, regardless of if the anticipated thing actually occurs or not.