The Dursleys goggled at him for a few seconds, then Aunt Petunia said, ‘You're a nasty little liar. What are all those—’ she, too, lowered her voice so that Harry had to lip-read the next word, ‘—owls doing if they're not bringing you news?’
‘Aha!’ said Uncle Vernon in a triumphant whisper. ‘Get out of that one, boy! As if we didn't know you get all your news from those pestilential birds!’
Harry hesitated for a moment. It cost him something to tell the truth this time, ...
Harry Potter and the Order of the Phoenix
I'm not sure the exact meaning of "Get out of that one" here. It probably means: explain that. How should we understand it correctly in this context?