Only JVDS was with her when this all went down, so we will never know for sure. But we may find some clues when we look at the murder we have more detail on - that of Stephany Flores. In his confession about killing Flores, JVDS attempts to paint Flores as the initial aggressor, which is of course almost definitely false. So describing his handiwork while minimizing his own culpability would not be unfamiliar territory for him. With Flores he assaulted her, then strangled and finally smothered her. Natalee's symptoms as described here are horrifying, but it is worth pointing out that while they may be consistent with overdose/seizure response as is claimed they could also be consistent with a TBI inflicted by violence or with strangulation. So to me, it almost "fits" that JVDS could have used the same methods of violence he would later use with Flores, and minimize that in retelling (she overdosed).
Or it could all be a hoax.

But I truly hope for the sake of the family they find the answers they need.