It's not clear to me if the author actually understands how staging works and how that affects the commands they suggest. They claim that stashing takes staged changes and then note that after stashing the working tree will be clean. That suggests the post is being dumbed down or the author is misunderstanding things.
Git guidance should almost always start with a brief introduction to the concepts you believe the audience doesn't know or fully understand. To begin with reset and not properly explain the index will just end up being confusing, especially with brief references to hard, soft and mixed resets later on!