It’s because they could see some parts of the game during the announcement trailer, where it shows some BotW “gameplay elements”, like:
- Climbing walls/mountains
- Gliding through the air
- Environment effects (grass burning, wind amplifies fire, ice freezes water, etc)
These things were available on BotW, and it also existed on Genshin. And the devs themselves said that the game was heavily inspired by BotW, where they took some notes about the things there.
Here’s the first trailer of the game, which was released more than a year ago.