I find this post thought provoking. I don't think it just automatically affirms our standard advice. I think it should make us think harder.
The data that we assume as our main reference points didn't exist then. The data that, in looking backwards, will make today's mistakes obvious may not exist yet either. In each case, the question is "what wise decision is logical using the knowledge of the time, in ways that can be replicated today with similar results." In 1919, from the graph presented, I assume that French houses in the aftermath of the Great War and a huge plague had very low values. I kind of doubt "buy a global basket of stocks" was a natural thing for the casually prosperous everywoman amidst the battered French countryside. I remember reading how 1919 involved giant strikes in which occupied Germans resisted French occupation by blocking coal shipments or something. What was that time's equivalent of today's "buy VTSAX and maybe a little VTSAX"? It probably wasn't "buy a lot houses that nobody wants", but that was the best technique.
In retrospect, buying some cheap houses would have been a great investment. But if they appeared to be logically super-cheap, who would have had the wisdom and capacity to buy them? What is today's equivalent of such a wise purchase? I kind of doubt that VTIAX is as cheap now as French houses then. Is the lack of obvious equivalent simply because we're not at a similar point in history? Even if so, is there something we should learn besides "our standard plan is fine, even though people in the past failed with something similar"?
It could be argued that the French case suggests "diversify across multiple asset categories, including something undervalued". Or "including real esate". Does that mean own rental property, or just a home? Or does it mean "buy commodities, because everybody hates them, which makes them like French houses in 1919"?