Most likely Jaguar is involved financially and it's not a straight purchase. From Wired:

"Jaguar, for its part, gets a large chunk of guaranteed sales and a chance to look like it’s at the forefront of this emerging technology."

- Waymo Expands Its Robo-Fleet with Electric Jaguar SUVs, https://www.wired.com/story/waymo-buys-jaguar-suvs/

Probably got them for free/really cheap. Waymo has regularly looked for partnership deals with car manufacturers, driven by the push to make Waymo look good on the budget sheets.

Also, companies pushing their own self-driving or ADAS solutions may be hesitant to partner with Waymo, excluding some of the large players.

Well, as long as the tech isn't ready to sell to consumers, 'partnering' with Waymo doesn't have all that much upside for automakers. It's not like Waymo is handing over the secret sauce or promising them exclusivity. I also suspect over the past decade Waymo has found fewer and fewer automakers fighting to make a deal with them.

Jaguar doesn't have a high-profile competing autonomy effort of their own, that would create a conflict of interest and/or invite spying.

The i-pace is electric, which makes integration easier and won't hurt for PR purposes.

And it's luxurious enough that no matter who you're showing the tech off to, it won't be a big step down from their personal car.