Ok, the well is now capped and under control, oil and methane isn't streaming up like it was with top kill. The oil flowing up diluted the killing mud, preventing it from settling out in the bottom of the well.
The well can be top killed again, this would be known as a static top kill, something done all the time in drilling after a BOP fires and successfully stops the flow. But in this case, the well is to be abandoned and since it had been completed, there is an annular space between the bottom casing (9-7/8") and the liner (7"). This space was left filled with mud when the liner was installed. There is no way to prove the mud is still there, it could have been replaced with oil and that would make the well flow if anything happened to the top of the well, like corrosion.
So that is why a top kill isn't enough. The kill well will be able to fill both well and annular space with cement.