It's interesting, I never actually thought that I needed to justify Dooku simply leaving after besting Anakin and Obi-Wan. My reasoning?

1. Dooku from the very beginning of the sequence only wants to leave. And if Anakin and Obi-Wan never arrived in time, Dooku would have simply taken off. He doesn't escape because he's in a pinch. He just intends to leave, regardless of who confronts him. The only reason he brings the pillar down on Obi-Wan is because he knows that he can't escape so long as Yoda isn't distracted. If Yoda never shows up, Dooku should logically just jump in his ship and get out of there.

2. Dooku doesn't show much interest in actually killing Anakin and Obi-Wan. Sure, he goes to chop Obi-Wan when he's down, but after maiming Anakin, Dooku looks like he's had enough, and actually turns off his saber - something he wouldn't do if he intended to still kill with it.

The fact of the matter is that within the context of the film, Dooku needs no reason to "escape". When Anakin and Obi-Wan find him, Dooku already intends to leave. He easily beats them, and he's not such a 2D villain that he needs to kill them afterwards, either. Dooku's face afterward is a "Phew, that was annoying. Time to get on with things" kind of expression.