The source is a report written by Julien Garran based on the difference between actual interest rates and an idea of what they should be called the Wicksell spread. There's a summary here https://www.marketwatch.com/story/the-ai-bubble-is-17-times-...

He figured there was a credit bubble like that around the time of the dot com bubble and now but the calculation if purely based on interest rates and the money can go into any assets - property, stocks, crypto etc. It's not AI specific.

He explains it here https://youtu.be/uz2EqmqNNlE

The Wicksell spread seems to have come from Wicksell's proposed 'natural rate of interest' detailed in his 1898 book

https://en.wikipedia.org/wiki/Knut_Wicksell#Interest_and_Pri...

I see, thank you!