I'm guessing this is what they're referring to- 121.436
(3) If serving as pilot in command in part 121 operations, has 1,000 hours as second in command in operations under this part, pilot in command in operations under §91.1053(a)(2)(i) of this chapter, pilot in command in operations under §135.243(a)(1) of this chapter, or any combination thereof.
When the FAA says "under this part" they mean operations conducted under 121 as the sentence comes out of part 121. If they wanted to say scheduled air carrier operations to cover foreign airlines they would have phrased it that way. Probably looking to make sure the Captain has a experience working under the FAA system. Things will come up in line ops they never covered in basic indoc.