It's a completely unnecessary assumption to make. Either Zimmerman stands by the story he made to police or he will attempt to change it during the investigation. If he does that looks quite bad. What his father is saying isn't going to change that. And in the end this is the father speaking, not Zimmerman. We'll know soon enough.
The originator of the ever changing stories is less relevant when comes to the surveillance video, as even in the original police report, he mentions that he was punched and hit.
His father takes it to the next level and paints a picture that would fall in line with the overall narrative of his son being on the defensive - being on the bottom of the confrontation, screaming for help, and all the while being beaten.
He mentions that the screams of help are the result of him being assaulted in the 'leaked' report, and those lasted for about a minute - which is probably why his father goes on to say "he was being beaten like this for nearly a minute" - but lets say the beating was only half that, and the other half was the supposed struggle - we know that the screaming was the start of the confrontation at least, and the screaming stops when it ends.
That would mean he was getting his head smashed from both sides for quite a while. Just count to 30 in your head and imagine getting assaulted badly for that amount of time. Would you come out looking like he did in the surveillance video? That absolutely does not add up - it didn't add up when he didn't get sent to the hospital, and it doesn't add up when you see that he was completely stable and
appeared to be completely unharmed.
With the ever increasing scepticism for his account of the story, I think people are just placing a higher burden of proof on Zimmerman, maybe unprofessionally, but not unfairly.
edit: And on the screaming - it doesn't make any sense. It paints Zimmerman as a fearful law abiding citizen who wants the help of the general public - even though he is the one who was originally at least -willing- to follow this stranger around for a while. So, what, Martin was some cool under pressure mobster? How could a 17 year old not be yelling and screaming too? This is of course an assumption, but does that kid look like someone who has killed before? Someone who is cold and unfearful? That doesn't match with the story his girlfriend gives of the confrontation, let alone the general history of the victim. Odds are, the 17 year old being stalked at night, once spotting a gun, would scream bloody fucking murder - not coolly pronounce Zimmerman's soon to be demise.
No, the screaming really only makes sense when you see that if there was a struggle, it probably wasn't as epic as it is being painted - that would give Zimmerman no reason to scream.