You are right that having a larger diameter makes the tube stiffer with the same amount of materials. That is one of the reasons why aluminium frames had rather fat tubes.
In fairness to Lexon, all titanium 24mm spindles had this design (flaw) because the milling of the inside of a tube is rather costly compared to the outside milling. (Guessing that the Elilee crank has been milled on the inside)
