Now Fred and George are very talented wizards despite what their O.W.L. results indicate; however I find it highly unlikely that they found a way to time travel without the use of a time turner. There is no indication in the series that there is an alternate method of time travel and any revelation of the twins using a different method of time travel this late in the series is not likely. There are more important loose ends that need to be addressed in Deathly Hallows than this. There is a better explanation for the Twin's time traveling.
How would they do it? The Fred and George who went to the match then go back in time and tell their other selves, the ones who haven't seen the match yet, who won? Or do Fred and George somehow go forward in time? We don't see anyone going forward in time in PoA, so we don't know if Wizards can do so or not.
If Fred and George have mastered this, without a time turner, this might be useful for Harry in the last book.
After finishing Prisoner of Azkaban Sunday morning, one of the details that I took for granted was how many subjects Hermione Granger took that year. Let’s count them off: Ancient Runes, Arithmancy, Astronomy, Care of Magical Creatures, Charms, Defense Against the Dark Arts, Divination, Herbology, History of Magic, Muggle Studies, Potions, Transfiguration. Twelve subjects in all. There was no way she could take all these subjects without the use of a time turner. This is a important fact to consider. We know that she left Divination forever around Easter and at the end of POA, she dropped Muggle Studies. Fast forward to Half-Blood Prince when she receives her O.W.L. results. She passed 11 subjects (10 Outstanding, 1 Exceeds Expectations), which means that she took exams for the 10 subjects she was taking and sat one "unknown" exam (probably Muggle Studies since she is a muggleborn witch herself meaning that you can probably sit an exam even if you are not officially taking it).
Hermione's prowess and academic excellence is very impressive when we consider that the most O.W.L.s you can take is twelve. We know at least two characters that achieved that, Bill and Percy Weasley. Considering what Hermione went through in POA, it is highly likely that Bill and Percy each were allowed to use a Time-Turner to get to their classes. I believe that Fred and George snuck into Percy's room and discovered his Time-Turner and "borrowed" it. They have a history of borrowing Percy's things like when they enchanted Percy's Head Boy badge to say Bighead Boy so it is not far fetched that they would do the same here. The only problem is that we have seen the Time-Turner go back in time not forward to the future. Three turns of the hourglass allow the user to go back three hours in time but we do not know how the Time-Turner can transverse the future or how many turns (in the opposite direction) it takes to go several years forward to the Quidditch World Cup. Despite these problems, the clues we have in the series regarding Percy's O.W.L. results and Hermione's use of the Time-Turner make the case for the Time-Turner theory all the more compelling.