I didn't put my actual inquiry in the comment since it would have made it too long. But I wasn't asking them about moving to Squarespace, I was very clear that I am burning a bridge with both of them and have no interest in being a customer of either of them. I told them I've already moved my domains out of Google Domains, and I wanted to clarify if any historical data about me and my domains (domain ownership history, purchase history, receipts, etc) would go to Squarespace. And they replied with what I put in my comment.
If I consider their reply to me, and the stuff I'm reading in the link OP posted, this isn't really a "transition," Squarespace is just buying the rights to all 10M+ domains Google Domains owns. But if Google Domains doesn't own a domain anymore, it won't be part of that transaction.
That's what I gathered, anyway. Hopefully they can be less ambiguous before the transaction actually happens. It will probably take the better part of a year, so there is plenty of time.