Those children were extremely neglected, but they still had exposure to some language. Just because they created their own language does not mean it happened spontaneously simply because of their human genetics, and their interaction with each other. I’m sure there would be some communication between humans if they were completely isolated with developed culture for their entire lives, but there’s no reason to assume it would be more than basic invention.
It could be said that inventing is instinctual, I guess, especially when it comes to language. We learn language very well, but language skills themselves seem to be very basic instinctually, if they are there at all. I think the reason this seems unlikely is because language spreads so efficiently from mind to mind. I think language skills express thought that we innately have, but that those skills themselves are constructs from that thought and not there to begin with. If that still qualifies as instinct, then sure, there are instinctually language skills. I still think the instinct is to create language skills, and not the skills themselves. A baby crying for attention, is a part of instinctive communication, sure. But an older child asking for a specific food they have a craving for because they feel a viral infection setting in is not an instinct, it is a development upon instinct that is aided by cultural influences.
I’m just very resistant to the notion that human nature is “meant” to have certain aspects to it that are clearly just cultural entities. It’s that way of thought that has, and continues to, destroy so much original human culture; both cultures of the the past that have developed over thousands of years, and cultures of the present that don’t get a chance to really start because they’re immediately considered to be against human nature simply because they don’t adopt certain norms that are considered natural because they’re found in many widespread cultures. It’s something that has to be kept in check, and it’s difficult to do with language, because it itself is a part of the problem.