MOZ drives me nuts. It is not on my short list of favorite sites. Sorry.
You have not linked to the resource that you are reading and some of your terms are a bit confused. For this reason, I cannot directly address what you are referring to. That is okay. I will go through some of this for you anyway.
First of all.
Stop thinking in terms of keywords. Google does not think in terms of keywords. This is one of the ways MOZ drives me nuts. I just went through this on another question. Search is about whole language. I keep repeating that dogs make simple word associations and it seems that people think Google is only slightly brighter than a dog. Google does not match search terms to keywords on your site. That is not how it works.
It is about semantics. Semantics is simply understanding language and the written word. It is about analyzing what has been written in detail that you cannot imagine.
Be that as it may.
What you are using in your example, is not a URL really. It is one fraction of a URL. A URL consists of four parts; the protocol and address of the site such as HTTP and domain name, the URI as the hierarchical path such as /path/to/my/page/, the file name such as robotics-research.html, and paramters such as ?q=robot-leukemia. There is more to it of course, but this should serve you well enough.
When it comes to the URL, each part is more important from left to right. In other words, the domain name is most important, the path second, the file name third, and so on. Of the four, most of the effort is put into the domain name and path.
Ask yourself, what is my site about? Ignoring branding a company name, any semantic clue should exist within the domain name and path. While the domain name is typically short, that does not mean that heavy semantic clues cannot be used. If, for example, your website is about "cotton fill mattress", you may be able to use all of these terms. However, if your website is about SEO, then competition becomes more difficult. You then have to think, What about SEO? What is my niche? What value do I bring that is different? For example, if you are an SEO contrarian, then that could work for a domain name. If you are an SEO instructor, then you may also use this for a domain name. Bob bakes cookies works too. So does grandma Hattie's recipes. Are you getting the picture? If it helps, you can write a series of short sentences to help inspire a domain name.
The same applies for paths and file names. Paths and file name apply to the content page itself and should describe the page as much as possible.
About the home page.
Your home page is always /. Period. You have seen some sites redirect / to a /path/filename.html and build their site from there. However, I do not recommend this. For your home page, you should not be trying to keyword load your URL with a path and file name. There are plenty of other clues for your home page. If you try and apply a keyword to the path of your home page, then you are applying that keyword to your entire site. It skews your results.
People often think that their home page should rank for terms. Quite the opposite. Your home page should be about your site and not rank for anything in particular except for a brand name if you have one. It is your content that should rank. Let your home page be what it should be and not worry about it. It is backwards logic. Create the content and let the site develop around that.
