A metaphor for what? A metaphor has to refer to something.
It seems obvious to me that 'pigs' would be a metaphor for Roman customs. Or maybe Greek customs. Since pigs were forbidden to the Hebrews, for whatever reason, the mention of pigs would immediately bring to mind the ruling nonHebrews whatever nation they came from. Since Jesus lived when Rome ruled Judea and Israel, then pigs refers to the Romans.
A Christian believer could extend this to his belief system, if he wanted to. Maybe there was a divine Jesus. He performed an real exorcism with a real man called Legion in which real pigs were drowned. However, He choses the details of his miracle. So maybe the Divine Jesus decided to off pigs as a metaphor.
The resistance to consider such possibilities may be pointing out some down to earth prejudice. The Christian may prefer to blame Jews rather than Romans. So he doesn't want to consider the idea that maybe the pigs were a metaphor for Romans.