According to this, the problem with grinding materials and re-melting them is that the

@NathanielStenzel Did you watch the video? The whole point is that they don’t regrind the material before printing with it again. If they were using fiber-reinforced material, that would totally explain why they don’t want to regrind it, but the prints in the video don’t look anything like a CF print. They do look like a print with a low-concentration metal filler.

Based on the appearance and the mass considerations, my guess would be aluminum, but that would probably rule out induction heating. Microwaves should work, though.