I don't think it's a breach of 1NF.
The "chain" icon next to the column name suggests that it is not a "real" field, but rather some kind of linked field that shows entries from the "Purchases" table for each user.
The lack of backlinks is one of the big frustrations I've had in other DB apps, since "this thing has many of these" is such a common pattern.
I've spent quite a bit of time plumbing Airtable's APIs, and am currently developing an app around Airtable to address some of these issues (zemanir.com) in case anyone is interested.