I'm going to say that's a bit of a misprint. Firstly demisemiquavers are "filled" not open, like a minim.
I think that it's supposed to be a tremolo between G and D for the length of a dotted minim (3 beats) so in fact your piano player is playing it correctly. A tremolo like that should be illustrated with the lines between the two notes but not joined to the notes. The other reason I think it is a misprint is that with the quaver triplet at the end, that leaves 3 beats in the bar left. The dotted minim has been used twice when it should be two dotted crotchets. This is what I'd play anyhow.
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