It is absolutely infringement if you paint a picture in Ghibli style. You just have fair use to infringe in a personal, noncommercial, educational, etc. purpose.

Fair use is a defense to infringement, like self defense is a defense to homicide. If you infringe but are noncommercial, it is more likely to be ruled fair use. If Disney did a Ghibli style ripoff for their next movie, that is clearly not fair use.

OpenAI is clearly gaining significant material benefits from their models being able to infringe Ghibli style.