Isn't it possible that the officers told SA that it was in the 911 calls/report, and they continued to pressure and he said he might have threatened HD. Since I didn't hear it myself in the 911 calls released I am inclined to believe that he threaten BD and CD but may not have threatened to harm HD. We know that LE is allowed to lie in interrogations and also there are many false confessions. Having said that, I think BD should be afraid of SA and not trust him given all the other details in this case.
One thing to remember is that, when researching SA's background, LE probably did not go to the 911 calls (at least right away). They probably looked up police reports first, IMHO, since they probably have that more readily available at their fingertips.
And if it's not in there, then my theory is this:
The threat against
Billie:
I think that if you were to tell 911 that your BF was threatening your life, they would take it up as a dispute between two adults - and give the complainant the option of pressing charges or not when they respond to the call. I'm assuming that BD did not press charges, unless there's an arrest record out there.
But moving on to the possible threats against
Hailey:
HOWEVER, if someone were to call 911 and say that their BF threatened to kill their child (either on the recorded call or when LE responded in person), then would the adult have the option to press charges or not, or would LE and/or CPS take over at that point? (Can someone more familiar with threats against children clarify?) Perhaps BD didn't report the whole story at the time for this reason and SA later 'fessed up during the recent questioning.
In addition, based on the reaction of the LE in this case, I would not be surprised if SA did in fact threaten the life of HD and yet it didn't make it into the written report b/c of the 'hassle' it would be - more investigations, more questioning, maybe CPS involved if it was decided that he was in fact living there, etc. (I know WS not into LE-bashing, but IMHO after the tape was released, it seems that the one officer is fair game to some degree regarding the seriousness with which he took all of these threats.)
Still drinking my first cup of coffee so I hope this all makes sense. And it's all speculation and MOO.