I would just add:
(1) Granger-causality is not causality as is typically understood, and is best interpreted as “predictability.” Thus, if knowledge of speculative positions does not improve our price forecasts, it is reasonable to conclude that speculation does not affect prices.
(2) Speculators can profit either by taking long (buy) or short (sell) positions, so there is no reason to believe they would have an interest in strictly rising prices.
(3) Prices have gone up for some commodities for which there are no financial instruments (i.e. speculation); and prices have not gone up for some commodities for which there are financial instruments.