I would argue that official Neo art has had such a wide range of styles that it could be mistaken for official art.
Mmmnn... Kuitsuku's image is still in an artstyle you wouldn't find onsite. Like it's very clearly fanart. Look at the weight of the lineart, the size of the eyes, how it's shaded, how those eyes are drawn, mouth size, ear size, tail tip shape, etc. Usually when it comes to drawing characters for companies you usually have to follow a set of guidelines so the artstyles are consistent. Let's take cartoons as an example, if the artstyle unintentionally changed multiple times during the episode then it would of course make the animation look incredibly unprofessional and be clearly noticeable to some viewers. Even in items featuring Korbats which are drawn in a different style you can see that (nearly all) of them follow a similar sort of "pattern". Eye placement is nearly always in the same place, ears are nearly always the same size, etc. Of course some items switch it up a little. Changing your artstyle can be hard shit and if the item looks good then I'm sure they thought "oh man whatever this is good enough", but the changes are never as drastic as the ones between the official Korbat image and Kuitsuku's image.
Maybe they also thought it looked official enough to pass as their own creation meaning they could get away with it. Since if it looked official enough then surely other people would think the same, right? There are plenty of people who have an untrained eye when to comes to art, who lack the "ability" to tell the difference between artstyles as they aren't as familiar with art as others. There are plenty of people who have never seen Kuitsuku's art too so they wouldn't think that it was stolen. But.... when you're provided proof that it's a stolen image then you don't just cover up your ears and cry saying that your in-house team created it!! You don't just lie and say they would ask the original creator for permission to use the image when they didn't. Even just a halfhearted apology like "haha oh oops looks like that certainly is someone else's creation sorry lol we'll remove it/give credit in the item's description" would have been good enough.
For varying styles used in items though... It makes me wonder if they've stolen art in the past, and just how much if they have. Someone on deviantArt is saying that they stolen an adoptable she created and put it on a book item, but she hasn't said which adoptable/book it is yet so we'll have to wait and see if this is true. But if they have then that would definitely explain the inconsistent artstyles used.
I believe this person was super famous (like for 7-8 years) and people would constantly steal from her.
This is why people are bringing it up.
Heh, she's still popular ^^ She's the one who drew these popular Disney/Pokemon images and does a lot of Aywas art. I'm guessing people don't steal her Neopets art as regularly as they did back in the day because she just... doesn't really draw Neopets art anymore hehehe