Why does xcrun think Safari doesn't support manifest.json keys?

Question

I am converting a very basic WebExtension to a Safari Extension, and am getting an odd warning when I run the xcrun command.

xcrun safari-web-extension-converter

Warning: The following keys in your manifest.json are not supported by your current version of Safari. If these are critical to your extension, you should review your code to see if you need to make changes to support Safari:
      manifest_version
      js
      name
      matches
      version

{
    "name": "Search Engine Detector",
    "version": "1.0.0",
    "manifest_version": 2,
    "permissions": [ "*://*/*" ],
    "content_scripts": [ {
        "js": [ "js/Bing.js" ],
        "matches": [ "*://*.bing.com/*" ]
    }, {
        "js": [ "js/DuckDuckGo.js" ],
        "matches": [ "*://*.duckduckgo.com/*" ]
    }, {
        "js": [ "js/Google.js" ],
        "matches": [ "*://*.google.com/*" ]
    }, {
        "js": [ "js/Yahoo.js" ],
        "matches": [ "*://*.yahoo.com/*" ]
    } ]
}

The extension works as intended when built.

Why is the command line tool giving me incorrect information?