I’m not sure what overspoke is supposed to mean by those who use the word. When you overeat, it means that you’ve eaten too much. When you oversleep, it means that you’ve slept too much and missed, for example, an appointment. So does overspoke mean that you’ve said too much, as in you’ve used too many words?
This is yet another example of the disconnect that can occur between speaker and listener when the speaker makes up a word instead of using perfectly good veteran words that are part of the English language.