Short-Term Investments for Enterprise Value

Mergers and Inquisitions says that enterprise value is the value of net operating assets to all investors. When they show theoretically what to subtract when moving from equity value to enterprise value, they count short-term investments as non-operating assets, which makes sense. Why is it that we only subtract cash when moving from equity value to enterprise value(EV formula)? Shouldn't we subtract short-term investments too? Or is the EV formula just a simplification to not make things too complex?

My understanding is that an operating asset is something that is needed to continue main business operations and generate revenue. Technically, some amount of cash will be needed to continue operations, but it's virtually impossible to figure out what that would be for each individual company, so we assume that the whole cash balance on the balance sheet is non-operating for simplicity. Is that correct?