No xp. They did not defeat Strahd
The DMG explains when to award experience points on page 260, emphasis mine:
Each monster has an XP value based on its challenge rating. When adventurers defeat one or more monsters-typically by killing, routing, or capturing them-they divide the total XP value of the monsters evenly among themselves.
The goal of the ritual is to
Awaken Wintersplinter so that it goes to destroy the Wizard of Wines Winery
This is the druids’ goal. The PC’s should get full xp from all of the druids and berserkers, whether they were slain, captured, or ran away, for by interrupting the ritual, the PC’s have defeated them and their goal.
However, the completion of the ritual and destruction of its target is not Strahd’s goal. Had Strahd wanted to achieve this end, he could have done so long ago; he could easily do so still. The PC’s have done nothing to frustrate his plans, and should get no xp for defeating him.
In fact, this dust-up with the players is part of Strahd’s plans. He wants to learn their abilities and assess their power. If they are too easy for him to beat in battle, he will be disappointed. If they actually challenge him, he will find them far more interesting. Whether he withdraws from the fight or not, he has gotten what he wanted from it. In no way have they frustrated his goals or defeated him.
From time to time, strangers from faraway lands are brought to his domain, to play the vampire’s game of cat-and-mouse. Strahd savors these moments, for though these strangers offer him no lands to conquer, they aren’t so easily destroyed and therefore provide a welcome diversion.
As a DM in this situation, you should respond as narrowly as possible to the players’ questions:
Player 1: “Why didn’t we get any xp for beating Strahd?”
DM: “You get xp only for defeating opponents.”
Player 1: “But we did defeat him – he had to flee.”
Player 2: “Are you saying he chose to withdraw? Or that he wanted us to win?”
DM: “I’m saying you did not defeat him.”
Player 1: “This is like Vallaki all over again – what does Strahd really want?”
One of the grand themes of Curse of Strahd is that Strahd has very complicated, intricate plans that involve the PCs, but they don’t know what those plans are. They spend most of the module trying to figure out why he brought them here and what he wants from them, when it is obvious that he could simply kill them if he wanted. Denying them xp when he strategically retreats emphasizes the theme that they really don’t know what is going on.