He could, but he will also have heard from his men (those dumbbrave enough to return) that the attack was staged by the Dread Pirate Roberts. It's quite likely that Humperdink would put together the facts at hand and realize that Wesley was the Dread Pirate. He may even have deduced the whole rotating name scheme. Humperdink, being an intelligent man and knowing that the man in black navigated the Fire Swamp, defeated Vizzini and his men, and staged the assault after dying and raising from the dead isn't likely to press his luck.
Besides, he lost a lot of face when his wedding was broken up (in front of the guests!), and he was eventually found in his own bedchambers, tied to a chair. Especially in fuedal societies, the nobility's ability to command respect/fear was correlated with their ability to rule. Humperdink could count on neither from his nobility or the common peoples.
Further, if Humperdink had pushed on and blamed the attack on Guilder, he'd be starting the war in a much worse position. He'd have no evidence, only supposition, and his military would be missing a skilled leader, as well as have reduced confidence due to their recent losses.
Lastly, if Humperdink had blamed Guilder anyway, all it would have taken is a statement from the Dread Pirate Roberts that it was not Guilder but himself who had staged the assault (possibly intimating that he'd been handsomely rewarded for it, or that Humperdink had slighted him) and Humperdink would have lost all credibility for his war, lost the ability to frame Guilder in the future, and made an enemy of the most famous pirate on the seas.
It isn't clear what the factors driving Humperdink to desire a war with Guilder were, but the many new downsides would have been a strong motivation to not go through with it.
