Legs, re loose ends etc...
It is a technique I use myself, because of my style. I start with building a character, then tell a tale from their point of view, and (almost always) in first person. That means, to steal from your quote, that they are in the dark about anything they don't witness and get informed about. That means other characters, built the same way, have their own lives and stories. From my own point of view, that means I am given opportunities to tell those tales in a subsequent novel precisely because of the ambiguities and loose ends.
I suspect that Mr Handler may be working the same way, as a sales technique. I might, of course, be wrong. After all, I just write romances!