I honestly find that hard to believe because you're not accounting for the other elements to the souray era powerplay. Markov/Subban I remember being a force but having zero help. It was just them walking the blueline. Either way Markov was an elite player.
Not sure what you find hard to believe, I'm providing you with facts, feel free to fact check if you're still unsure.
You said they were dynamic together on the PP, they were not. That's not me making it up, they just weren't that good together on PP, certainly not as good as Markov/Souray were (albeit for a shorter sample). The Habs PP was 10th, 5th and 1st in 3 consecutive years with both Markov/Souray on the point (Kovalev also a factor obviously).
I remember Markov and Subban being much better at even strength and that's probably where you might be mixing things up.
I do agree though, Markov was an elite player…my favourite player on the Habs during those years and probably in my top 5 favorite Habs players all time.