And yes, the defense attorney could put him on the stand and he may relate exactly that.
And the Prosecutor will try and punch holes in it.
If they can't punch enough holes in it to discredit it (or in other words, prove to the trier of fact) his version is going to stand as is.
As it is right now, we are discussing what is known.
The only witness said there was banging. That is the evidence that we have to go on. Period.
The Prosecutor said in her press conference that "It's alleged she was shot to death by the home owner after she knocked on his locked front screen door." The only folks who had alleged such has been the family.
The only evidence that would support knocking, would be an eye witness to such, yet none are present at this time.
The only other evidence that may be available, is of her touching the door. Which really would not indicate whether done by knock or bang.
All we really have is the eye witnesses account that she was banging on the door.