What is the takeaway here that he knows Hank's name? Simply irony?
Pretty much, yeah.
Creed constantly forgets everyone's name in the office and in the one time that they need to know someone's name, Creed actually gets it right but is completely disregarded.
Examples of Creed not remembering coworkers and/or their names:
- Meredith: S04E02, refers to her as "the chick that [Michael] hit with his car"; S03E17, calls her "Mary Beth"; S02E21, calls her "the redhead"; and in S03E02 introduces himself to Meredith as if they've never met before.
- S05E16: calls Andy "Jim" when giving dating advice.
- S07E11: refers to Holly as "one sassy black lady".
- S02E21: refers to Pam simply as "the receptionist".
- S03E02: refers to Angela as "Andrea, the office bitch".
Creed knowing Hank's name is a bit of comedy that's not really picked up on unless the series is watched multiple times and/or the viewer is really paying attention the first time around.
It's even funnier that out of everyone in the office he forgets Meredith's name the most, given that they sit right across from each other.
Are the two of them close for some reason?
Though it's never shown, if Creed and Hank were friends then they'd have probably bonded through music since the two of them play guitar/sing. Also, they could have crossed paths frequently since Creed was once shown flying a drone on the roof the building (S08E09), as well as living in the office (though that was in the last episode of the series(S09E27), he could have been doing it in the past too).