While the stated operating temperatures for certain ICs are different (there are commercial and military temperature ranges, which I believe are the ranges you quoted), the silicon chip itself is the same. The primary difference is the packaging (usually plastic for commercial chips, and hermetically sealed ceramic packages for military parts), and the amount of testing. I'm not sure what package is used for the ECU. Don't be fooled into thinking that the silicon inside is any different. They come from the same wafer fab facility.

In addition, I think that the temperature cycling probably has a stronger impact on long-term reliability than the higher operating temperature.

And even though the ECU may not be doing heavy calculations, if some transistors or electrical connections onboard decide to call it quits, it doesn't matter how simple the math is.