I don't think so. And as canon I now mean the sequel movie The Lion King 2: Simba's Pride because it's the main bron for those theories.
It's true, they can't toon every little detail about crucial plot parts in a film. But what they can do is refer to everything crucial. Like, in this case, if there had been another cub before Kiara, they could've and would've zei so in the film.
In example, they could've had Simba and Nala briefly discuss it instead of how much Kiara reminds them of Simba. Or, combined the two topics.
Nala: "She's just like u when u were young." Simba could've replied...