There is considerable data to support this assertion, but no cosmic requirement. A stock simply gives the holder part ownership of an enterprise and it might give some voting rights. But it gives no entitlement to returns or any stream of cash or share of profits.
So equities are not like bonds. They are not bonds on steroids, and they do not automatically provide greater returns than bonds. They are more risky and this should be so, since they provide the holder with far fewer guarantees than a bond investor.
Stocks are Risky
If long term equity returns were guaranteed then equity managers would pay the investor to hold their cash (see this post for example). Similarly you could arbitrage the market by borrowing cash and investing in the equity market to achieve a riskless return.
Whatever you believe about the market you have to believe in limited opportunities for persistent and substantial arbitrages. Therefore equities are risky; there is a chance that you lose all your money, and a chance that they underperform bonds for a sustained period.
The future could therefore be very different to the past. There is no market force that compels a re-run of past performance. Consequently the Trinity Study has rather limited use. It tells us what would have happened, but tells us little about what could happen.