I guess it seems clever, but also it seems like it wouldn't make any sense. I mean an obvious comparison would be some of the things on this list http://medieval.ucdavis.edu/120D/Money.html

but how does it even work - what things on that list have actually remained unchanged? Is a chicken in medieval times really the same as chicken today, and that's just in the last 800 years, not in the last 5000.

And in what direction do you compare? I suppose the price going up shows the growth of the economy, but what does the price going down show. Let's take aluminum, that price would have gone down a lot since medieval times, because it has been very much changed by increase of technical abilities. So we must not look at that because it is no longer like the aluminum of medieval times, but if you can not look at and measure stuff like that how can you actually measure the growth of the economy where value creation takes place.

I would need to see some arguments on how this works to even begin to get past the sounds clever but also sounds wrong stage of thinking.

for example there might be some theory somewhere that the way to measure the growth of the economy is to take something relatively unchanged since ancient times, like a chicken and then calculate the amount of biomass change taken up by chickens since that time (this is a spur of the moment wrong idea here) but some animals have increased in biomass and then decreased because the demand for them has not continuously risen over time - for example: horses. If one used the biomass idea then it would see the economy increased steadily from ancient times to near modern and then there was a startling depression that wiped out several hundred years of economic progress.

Of course nobody is doing this silly thing, but trying to think about how the way it is being done works seems to be equally as silly to me.

One counter-example would be the cost of health-care.

That's a terrible example in the context of economic growth over millenia for the simple reason that anything resembling modern healthcare basically didn't exist as recently as a hundred years ago.

Yes that is the point.