It is called a "figure of speech".
It's a little bit surprising to get such pushback on a genuine question when there are substances you could describe as "electricity you can put on the back of a truck" such as hydrogen canisters and various forms of battery!
But then there is this: https://en.wikipedia.org/wiki/Aluminium%E2%80%93air_battery so who knows.
As others have said, I was mostly referring not to the ability to easily convert to and from aluminium, but instead the fact aluminium is easily transportable and such a useful material that there is rarely surplus (if aluminium were cheaper, it would take over lots of places where steel is used, and even some uses of concrete).
That in effect means that if there is excess electricity in a region, it can in many cases work out cheaper to build a decent road to the region and build aluminium smelters than to build power transmission lines out of the region.