I would assume the same thing, but none of the other SMB power ups are on the chart. The "Atom" is used in one level and may actually "kill enemies". I didn't do a whole lot of testing, but I do rember some strange behavior when I was researching the power ups.
When you grabbed the atom, if there was another power-up on the screen, it'd disappear and you'd get it. Now, if the game is coded such that grabbing a power up and killing an enemy use a shared code path, this could simply be a side effect.
If that's the case, it kills everything on the screen, or within a given distance, or something. I'll have to test this theory out and see what happens.
I just tested it out and I've confirmed that it seems to kill all enemies on the screen at the time you grab it. It appears about halfwaty through 4-1 in an area that's pretty heavily populated with enemies. Also, there is a Lakitu on that level which could lead to even more enemies about by the time the player gets to that point.