This scentence (spelling deliberate) is linguistic sleight-of-hand. I doubt mightily that a volcano ejects all of the available magma. It reads like it's a prediction, but the purveyors know it's not.
It would be hilarious if it did. I can just see the Earth flying around the solar system like a punctured balloon as it spews out all available magma. That would be a fitting end to our civilization, species, and world. To go out like a giant whoopee cushion.
On that scale whether a volcano ejects all, or most, or just some over a period of time will make no difference worth noting. Few will be around to note it in any case.