I'll say again; "A young Jedi named Darth Vader...". There was no young Jedi named Darth Vader once Luke's father becomes Vader. So the Vader as father 'twist' is not justified by metaphorical truths. The only way it can work with what was said is to understand that Obi-Wan is an out and out liar. Its not a plot twist, its a plot change.