Video proof
I don't know, but doing Battle Butt in that same situation instead of Pummel Horse activates burst (Do I have to make a "Battle Butt cares too much" vid...?). So it's definitely Pummel Horse.
Pummel Horse is still active for a bit after that last punch though.The last hit of Pummel Horse is a punch. And I'm pretty sure I remember Mike saying anything that's a grab isn't supposed to activate burst, when I found out that Pummel Horse didn't activate it. I was assuming it extended to Excellebella, which was also a hit grab (there was an early bug where if you did it too fast sometimes it'd activate burst, but I thought that was different). I guess it's fine as is, because, I mean, when are you ever going to set this up. But still.