Like the Enhanced Edition of D:OS, the Definitive Edition of D:OS 2 is a separate release, with various updates, changes and optimizations. It is pre-full-release.
There will be updates during the Xbox Game Preview period.
Early Access on PC was also just the first act.
There is no standard for Early Access pricing; some companies offer a discount, some have it more expensive (to match a Kickstarter price point, for example). With D:OS and D:OS 2, the price during Early Access was/is the actual release price.
Early Access is for getting feedback, balancing information, etc:
Divinity: Original Sin 2 Spotlight | Meet Feedback Billy.