But there is no other possibility left so yes disney is lying because otherwise people would freak out that Kiara and Kovu are family (funny thing is you can say the same about Simba and Nala)
well Timba and rogueprince Kovu's father couldn't have been a rogue since lions live in prides with one or two males and for an order a rogue to mate with the females he has to fight with the males first(and win) and also if there was a rogue who did it in a sneaky way scar would have never chose Kovu to be his heir because think about how angry you would be if suddenly Your wife(i think that Zira was Scar's wife)would do it with another person?
The Outlanders don't HAVE a male, Scar is dead and at the time Nuka was too young to mate with anyone, especially his own mother. Kovu, furthermore, could not possible be the spawn of those two, seeing as he has physical attributes unique from either...
But we all know that Zira and the outlanders where banished so this means that Scar could have been Kovu's father. And we don't know Nuka's age but i think he was an adult since when Kovu is young he still looks the same as when Kovu is old so in that time of years he didn't change which means he was already adult when Kovu was born