How do you "defend" your category in a way that serves the buyer?

You do need to walk a fine line between highlighting that fact that you’re unique while acknowledging the other choices customers have. There are a few considerations here -

• As part of the category creation story, consider highlight some trends in the market (or shifts in user behavior) that play into your key strengths (e.g. if you’re selling an AI solution, play into the trend around how more user data is being generated than ever - where only AI can make sense of this data). This helps to bolster the “Why you” part of the story in a world where there are many incumbents.
• Be up front about the choices customers have today and what those choices are specifically good at and where your solution provides unique value that that they don’t
• Consider customer stories and examples - focus especially on why customers needed to go beyond what the incumbent solutions were providing and the specific value that your solution brought to bear on their situation

Companies create and “defend” categories because they believe, or want you to believe, that they solve existing problems in a new way or are packaging together solutions to a variety of problems in a way that wasn’t done before. It becomes a natural way to deposition other companies who fall into older/existing categories because they do things the “old” way or other reasons. That said, it's also a disservice to yourself (even arrogant) if you disregard the older categories altogether.

Example: At PlanGrid (acquired by Autodesk, I’ve since moved on) I led the launch of a new category we created called “Construction Productivity Software”. “Project Management Software” was an old, existing category well documented on Capterra and elsewhere and we certainly had competitors with lots of competing products and features. While we had features that fell into that Project Management Software category, we also had new features and capabilities that hadn’t traditionally fallen into that category (or existed at all), and wanted to educate the market and focus on the impact we could make on individual worker productivity in addition to larger organizational benefits (as opposed to “project” as the main focus). But you better believe we paid for Google ads against keywords like “construction project management” and related terms.

The best way you can serve the buyer is to 1) make it easy for them to find solutions to their problems and 2) help them realize problems that they didn’t know they had 3) make it easy to understand why yours is (or isn’t) the best to solve them. Categories matter for google searches etc. to make it easy for a potential buyer to discover you. But once you’re talking with someone 1:1 the category doesn’t really matter anymore -- you’re talking to them because you already believe they may have a solution to your problem, the sales person no longer needs to lean on a better known category to get your attention.