The EU isn't sending anything to the US, they're releasing bits to their domestic marker. Donnie hopes this will temporarily reduce global demand so he can claim diesel priced are falling for the midterms.
NASCAR Americans will believe it.
Also, you can't really truly stockpile meaningful amounts of diesel or refined petroleum products. They denature over time. You can store meaningful amounts of crude but crude needs to be refined before use. That's why there are no huge gasoline or diesel stores, and why releasing crude doesn't immediately lower prices. Managing refining capacity and avoiding peaks and troughs in production rates also means supply shocks in either direction can be really bad. See 2021 when crude futures had a negative price for a bit. You could be paid to take possession (not store, own) millions of barrels of oil. Gotta manage the production rate.