> The GDPR recitals do not recommend gathering additional data solely to be able to fulfill these requests
One, these are non-binding recitals.
Two, the conflict between (a) data-furnishing requirements and (b) advice against retaining data that would validate the requestor is exactly the point of this post.
So they can identify users for purposes of complying with GDPR? (For example, to handle the data requests highlighted in this post.)