The fewer vs less thing is a myth.

You're also using "myth" with a unique meaning here.

Both have been used interchangeable in English for a thousand years or more.

Not exactly. But also irrelevant.

They were never interchangeable, but prior to a couple of centuries ago, you could use less for fewer. You couldn't use fewer for less.

But this is irrelevant. We don't speak the English of 1000 years ago. Prior to 1066 is Old English. When a modern reader reads old English:

Hwæt. We Gardena in geardagum,
þeodcyninga, þrym gefrunon,
hu ða æþelingas ellen fremedon.

They don't think "ooh, look how they used 'fewer'". They think "what language is that?"

It was only the middle class Victorians who tries to make English fit with Latin who came up with a different jargon usage.

Yes. The rules of English are often decided upon. Nonetheless they are.