"Yet once more I will shake not only the earth, but also the heavens." (12:26). What does the letter say these words mean?

The words come after a warning: "See that you don’t refuse him who speaks. For if they didn’t escape when they refused him who warned on the earth, how much more will we not escape who turn away from him who warns from heaven" (12:25). The letter gives them as a promise, "whose voice shook the earth then, but now he has promised, saying" (12:26), and then says what they mean in its own words: "This phrase, “Yet once more” signifies the removing of those things that are shaken, as of things that have been made, that those things which are not shaken may remain." (12:27). And it goes on: "Therefore, receiving a Kingdom that can’t be shaken, let’s have grace, through which we serve God acceptably, with reverence and awe, for our God is a consuming fire." (12:28-29). Hebrews names no passage for the words of 12:26. The WEB's Haggai 2:6 has words like them, worded differently: "Yet once more, it is a little while, and I will shake the heavens, the earth, the sea, and the dry land" (Haggai 2:6). These notes don't say when the shaking is, apply it to no event, and name the one who speaks no further than the letter's "him who warns from heaven" (12:25).