They don't know it was defending the home, there is in fact at this point in time (the time of arrest) no evidence beyond the suspects initial statement to suggest that it was.
A body has been found stabbed in the street, this guy has admitted doing it, SAYING that he had broken in and had 3 accomplices who carried him away after he'd been stabbed.
The assumption of the situation has to be reasonable worst case, that he has gone into the street and stabbed this guy. For which he's obviously arrested.
Again there is NO evidence at the point of arrest to suggest he is acting in defence of his home, primarily because the body is nowhere near his home.
And we're back to ... presumption that the householder used unreasonable force.