Source please? The May 2020 Form 4473 which is the one currently in use has no such requirement in Section C - Question 26.a. of the instructions:
Question 26.a. Identification: Before a licensee may sell or deliver a firearm to a the person, the licensee must establish the identity, place of residence, and age of the transferee/buyer. The transferee/buyer must provide a valid government-issued photo identification document to the transferor/seller that contains the transferee’s/ buyer’s name, residence address, and date of birth. A driver’s license or an identification card issued by a State is acceptable. Social Security cards are not acceptable because no address, date of birth, or photograph is shown on the cards. Identification documents such as a driver’s license or identification card issued with binary, non-binary, or no sex designation may be used as an identification document. A combination of government-issued documents may be provided. See instructions for question 26.b. Supplemental Documentation.
If the transferee/buyer is a member of the Armed Forces on active duty acquiring a firearm in the State where his/her permanent duty station is located, but he/she has a driver’s license from another State, the transferor/seller must list the transferee’s military identification card in response to question 26.a., in addition to PCS orders as indicated in 26.c
Or is it just more dealer ignorance or making up their own rules to prevent a sale to someone they're nervous about?