I’d say it is helpful to know but I wouldn’t say you have to know. What may end up happening is that you will sooner rather than later be dealing with libraries that were written in Java. From the IDE you can look at their source code when wondering about how certain portions of a library should be used. So having a basic knowledge of Java would help there.
With that said, I think it is very feasible to start writing Kotlin and pick up other things along the way if needed.
What you link here is not an official documentation or guide for the Kotlin. It is one of many courses available in the internet. Anyone could write a course and while doing this they could assume you either start without any programming skills at all; you start with some basic skills with any language or with skills with Java specifically. In this case they assumed the last, that’s it.
Java is useful to know when you start learning Kotlin, but I wouldn’t say it is necessary. However… there is another reason why Kotlin may be not the best choice as a first language. Kotlin is relatively new language and while it is mature enough, I’m not sure if guides for it are good enough. There is a big difference between learning the language X and learning programming using language X. I believe most Kotlin programmers used other languages before it, so existing guides are probably more about learning Kotlin than learning programming using Kotlin. If you are looking for the latter then it may be not that easy to find.
I understood your reasoning. It makes sense. I think it’s it.
In my case it is not the first language. I program in PHP functionally. I’m learning Object Oriented. Hence the doubt in learning about Kotlin. If I learn Java due to “object orientation” or if I already learn kotlin and object orientation together.