Unsurprisingly, the AP article actually doesn't clear up the question of Walz's military rank. The statement from Walz that he retired as a CSM implies two things. First, that he was an E9, second that he had a special status as an E9. E9 is a
pay grade that comprises two separate
ranks: Sergeant Major (SGM) and Command Sergeant Major (CSM). The quotation from some retired COL in the MN Guard suggests that Walz actually retired as an E8. In that case, Walz's statement would be double false, for he'd be claiming not just CSM status, but E9 status. But for all the article says, it's possible that Walz retired as an SGM (E9), but not a CSM. In that case, there would be only the one falsehood. (This is all assuming, as the article seems to grant, that Walz was not in fact a CSM.)
It's perfectly possible to hold an "acting" position without holding that specific rank. In my small unit, our top NCO was a Master Sergeant (E8), but we always referred to him as our First Sergeant (also an E8 position). First Sergeants are oftentimes referred to by their troops (and officers) as "Top," and it's definitely a different rank from Master Sergeant, even though the pay grades are both E8. If our "first sergeant" were to claim to have retired as a first sergeant, that would be technically false, even though he more or less did serve as our first sergeant, and was treated as such. I frankly do not know exactly what the difference is between 1SG and MSG, despite my 6 years of service as an enlisted man.
I never got beyond E4 myself, and was happy to stay below the level of E5 because I didn't want the responsibility of becoming a sergeant. But these things do matter.
It's not a big deal, and I'm only weighing in because military pay grades/ranks can be confusing to non-service members (and as I just admitted, even a bit mysterious to former service members). I still don't know if Walz ever made E9, or if he retired as an E8, based on that AP article. One knows that journalists don't know anything, and one doesn't expect them to clarify anything. This article is a good example.
I would say, though, that it's quite possible that Walz retired as a CSM even if he didn't hold that rank while he served. They may conceivably have boosted him, so to speak, as a retirement gift. It could happen. It can also happen (and I think this is normal, but don't take my word for it) that if he was temporarily promoted to CSM while he served, but then stepped out of that position, that he nevertheless retired as a CSM, because you often retire at the highest rank you've held, even if you've been moved downward while in service. (Eg. If Custer had retired just before the Last Stand--not a bad idea, as it turned out--he might have retired as a General, even though he died as an LTC, because he'd been a general during the War Between the States. But again, don't take my word on this one. I never got to a point where I was thinking about retirement in the military.)