@Sweet_Seats Frankly I think you're nitpicking a bit too much. It's 100 million years, and he seem to have done it twice. Meaning 200 Million years.
Talent is composed by multiple factors but to mee these two are what matters. First the mindset, this is the part of talent that can be acquired. A talented person has a mindset that naturally allows them to more easily understand the concept behind something, but simply repeating and training that something will eventually allow a less talented someone to properly undertand it.
And there's the second part, the one that depends on innate characteristics, which is the physical predisposition towards something, a talented person is born with a body that can acquire the muscles and reflexes that suit a certain ability.
Usually is very hard if not completely unproductive to train something you're not talented at, you'll simply won't understand things fast enough, and your body will usually be underdeveloped for the task, which is what makes you commit mistakes and generally underperform. But this is only valid for people living within a normal timeframe. If you could live for centuries, talent will be worth less than garbage, if you could simply burn a couple of decades of constant trianing, you can brute force the aquisition of the necessary muscles or, since you'll have enough time to avail yourself, you'll come to fully understand the topic you're trying to study, once that understanding comes, there won't be any difference between you and the talented guy. More importantly since you have millions of years to study, and those millions of years go by faster than the time in real world, you easily can surpass geniuses, merely because time is no longer a factor.
You don't need the story to tell you that what the MC gained was more than just technique, physical conditioning, the proper mindset. THe MC is now a master swordsman in his own right. After all merely repeating techniques will already train your muscles, in real life it's an impractical way to train you body because this method is simply less efficient than training your muscles directly with the proper equipment, however he's been doing this for millions of years, no amount of equipment and diet can replicate this absurd timeframe. He is definitely several times stronger. Since he spend those years repeating movements and creating new moves, his muscle memory should be so impecable that he should be able to easily recognize any similar moves and react to them faster than anyone else can, with this I can assure you that he's gained a flawless technique, more importantly we've seen that he or the world he was in was capable of creating a clone of himself, of which he used to test all techniques in real combat, which means that his personal Style is battle tested and he has gotten rid of any moves that simply don't work. All the information you need to know was already properly shown to you.
Lastly, no he didn't forget his training, more than once the story shows you that he merely thought that his long years of training were a dream, because he couldn't accept the absurd reality of having spent millions of years training after pressing a button.
And there is also the fact that his extremely low self esteem make him doubt his progression. even though a healthy person should already know that they had become op. But that's why lack of self esteem is dangerous and requires help of a professional to fix