The indivudal you cite is NYPD. NYPD was informed by FDNY that they expected WTC 7 to come down soon. I doubt that they gave NYPD all of the technical details as to what they saw occurring, as far as apparent loss of structural integrity. It was not neccessary that they do so. What did matter was that the area was unsafe and that they totally did not want anybody within the collpase zone.
So here is a cop who knew that two buildings had already comer down. He probably had not had time to do a lot of thinking about what had caused the failuire of the towers, nore had it been explained in any detail in the news broadcasts. The closest thing he could have thought of to that dust plume was an explosion of some sort.
Do learn how the human mind works.
When your MO is to pick apart scenarios and events into their irrelevant and insignificant minutia, why would you bother with how the human mind works.
TAM