Whatever he was on, Ubereem was a monster back then. He would grab 260+ guys in the clinch and throw them like they weighed 180. As awesome as it was, he was definitely getting some extra "help" to be such a beast. It was shocking (or not, depending on how you look at it) how much strength and power he lost once he was 2-3 years removed from that phase of his career.
Regarding Brock's jabs, it showed what was always a weakness in Overeem's striking, he doesn't defend jabs and straights well at all. Despite his weak chin, he has always been better at defending when guys load up and throw hooks. He has a good shell defense to defend the sides of his face. But anything up the middle splits right through his guard. The Volkov fight is one of the few where I've seen an opponent focus almost exclusively on attacking that way.