I'm not sure war diminishes a Jedi's combat prowess but peace does strengthen them moreso, being that they are able to focus their connection in The Force and also hone their lightsaber combat. They can't really do that, if there is a war going on now can they?
Though Vent am curious as to the point where Anakin defeated Cin handily bit, now granted in like...the 2 second holo vid we saw, we see Vader choking a padawan and fighting against Cin one handed I don't see how that is a full representation of what went down. But....ya know, given that Cin is the Temple Battlemaster(making him one of the top duelists), who was trained by Yoda specifically in saber combat, trained Obi-Wan and Anakin and was the last hope in stopping Operation: Knightfall, I don't see how he wouldn't be a challenge for Anakin.
No this isn't fanboyism coming out, it just seems logical is all given who Cin is. Unless of course there is a source, saying otherwise but if not then I am none too sure.