Were Mwindo to remain in the womb for 22 months instead of 9 as usual, he'd want to be able to perform magic in-utero, since otherwise his failure to be born on schedule would be fatal for himself and quite likely for his mother too, for two reasons:
The first reason is that the human placenta has a maximum lifespan of a little over 9 months, after which it dies, and if the infant is not born, decay products will cause the death of the infant and likely the mother.
The second reason is that the mother's pelvic opening limits the maximum size of the infant, particularly its head. The size of the mother's pelvic opening is limited by the fact that the human upright posture causes the abdominal viscera, which in quadrupeds are suspended from the upper surface of the abdominal cavity, to collapse upon the pelvic floor, so quite simply, too big a pelvic opening, and the mother risks a fatal prolapse of the pelvic organs.
So, assuming Mwindo's magic can overcome those two issues, he has an extra 13 months in the womb. That's a good thing... isn't it?
When Mwindo is finally born, he'll be physically 13 months old. Surely that means that he'll be able to do things that 13-month-old children are able to do?
The answer to that - barring a lot more magic - is 'No'. By 13 months, normal human children may be beginning to talk and walk. However, they have had 13 months in which to work up to it. While Mwindo's peers who were concieved at the same time have had more than a year exposed to the world, what has Mwindo been doing? Hiding in his mother's womb, doing literally nothing.
What does this mean? Mwindo has missed out on 13 months of exercise and experience, all during the crucial first year of development while the brain is at its most plastic. Instead of learning to move, percieve the world and understand other people, he has been in a sensory deprivation chamber.
Research has showed that when an infant misses out on early experiences, their brains do not develop as fully as their normal peers. With the brain, it is literally: Use it or lose it.
So, what would happen to Mwindo? He would most likely be considered developmentally delayed even if his time im-utero was not considered. He would walk later, talk later, not ever see as well, nor ever hear as well as his peers. Perhaps worst of all, he won't have had experience of human contact, so he'll likely be autistic. Not Asperger's syndrome autistic, where he's socially inept but otherwise a genius, but severely autistic in that he is developmentally delayed in every area, and has fits or tantrums because the world is too much for him to deal with, too noisy, too bright, too busy. He will be good at something, though: being alone. He'll likely be happiest when put into a dark quiet environment for long periods of time, with little to no human contact.
So, unless Mwindo has a whole helluva lot of magic, he will be born to be not a cultural hero, a person of great significance, but an impaired, reclusive shut-in who, unless his society is very forgiving toward people with disabilities, will most likely be tormented every time he shows his face in public.
If Mwindo is born into Africa at pretty much any time up to and including the present, unless he is lucky enough to be born in a modern hospital, he may have a rather short life expectancy. I'm not sure if the OP wants to consider the high possibility of infanticide...
I'm sure that the OP wanted to give Mwindo the best possible start in life, but unfortunately this almost certainly isn't it.
Elephants do it, you might ask, so why can't we? Evolution is the answer. Elephants have evolved over millions of years to have 22 month long pregnancies. For elephants, they are normal. Humans have evolved over millions of years to have nine month pregnancies. Mess with that and it isn't likely to turn out well at all.