You're not wrong! But neither were they. All the serious calculations were being done with serious, built-for-purpose, statistical simulation software on dedicated hardware.
Excel was mostly just mucking around with summary data being prepped for powerpoint slides, and various hacky fudges that high command wanted but wouldn't budget for them to be built into the models properly.
Last I heard (I left that team a few years back now) they are moving towards a proper end-to-end database solution that eats text files and shits slide decks, but the whole environment is something like 40% compensation for inflexible legacy systems and 20% working around flaws in previous attempts so it's been slow going.