Lots of groundless assumptions on your part I must say. Maybe if you take like really the worst games you'd have something like this. Most Kagura releases easily trump Mirthals game. In fluent English with tons of gameplay mechanics.
Kagura is a publishing company. A major one as far as NSFW games go. Making this point twofold irrelevant, as not only they don't make games directly, but it's a whole team of people.
Sounds like a lot of cope to me, all these asian devs started somewhere. Acerola, Fullflap, Clymenia, Miel, Alicesoft and so on all make better, longer games and need much less time.
And that's just dismissing the point really. Most of these are still somewhat generic anime art, which is gonna be faster to make than the kind of stuff Mirthal does. I don't know enough about those developers to say with any certainty, but I don't think all of these are completely independent solo devs either.
Could Aurelia be done faster? Probably, we don't have surveillance over Mirthal's life to know exactly though. There are devs that do less in more time.
Another thing about japanese games is that for the most part, unlike the subscription models they are only paid for at release. There are several conclusions that could be made from that, but I ain't a market analyst to say anything with confidence here.