Why? He used different holds, and the only rounds that went low were when using the incorrect hold.
When he used the correct hold, he didn't shoot low. That's not really indicative of pushing the muzzle. What would be indicative of pushing the muzzle is if he were low on all strings, which is not what we see here.
In addition, pushing the muzzle usually results 1) in a much greater shift in POI, whereas this is minor and lines up exactly with the difference between using the incorrect and correct holds, and 2) pushing the muzzle usually doesn't result in such high consistency with the grouping.
As for the new info from Serpico about him using the Dot-over-target hold at 25, I'm also still suspicious that he actually is. What we see institutionally is lizard brain activity; a person thinks they're doing one thing, when they're actually doing another because their lizard brain takes over and they start reverting to what is ingrained. It appears this problem only started when he started shooting the Gen 5 with Ameriglos, just like we've observed institutionally. I'd be curious what his SIRT pistol's POA/POI is regulated to. Just like I guessed, "Hmm, I bet he's shooting Ameriglo Agent sights....", I'm going to guess that his SIRT is not regulated to a dot-over-target hold, and he's been unknowingly reinforcing the improper POA.
So, we know from multiple agencies that this is an observed phenomenon by institutional shooters when going from one POA/POI to another. What we see here is a guy displaying shooting characteristics that are consistent with that, and shooting that is not consistent with muzzle push in terms of severity and consistency. Occams Razor, and all. IDK. He probably just sucks and needs to be told not to push the muzzle, because that's obviously worked out so well over the last 6 years.