This is how I interpreted the original question and indeed it makes no sense, JavaScript from a website should not be allowed to interact with extensions like this.

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flomo 2 hours ago | parent | next [-]

It's actually the extension injecting itself into the webpage, often to interact with it. (I imagine much of this is just looking for global ExtensionName objects.)

	▲	angoragoats an hour ago \| parent [-]
		Actually, the article is clear about what is happening technically, and it’s both. Chrome does, in fact, allow the page to make requests for resources stored in the extension bundle, and this is one of the two fingerprinting methods that the article describes.

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encom 33 minutes ago | parent | prev [-]

>JavaScript from a website should not be allowed

Agreed 100%.