Mg as organic "chelated" salts (glycinate, glutamate, basically every amino acid, citrate etc) is extensively absorbed; however some fraction of the dose you take of a chelated Mg supplement is the chelating compound itself. Mg as oxide, carbonate or hydroxide is poorly absorbed.
I would imagine most magnesium in plants would be as some sort of chelate (either with phytate, bound to protiens, or some sort of amino acids), and would be well-asborbed.