> I don't actually understand how Markforged and BASF can print with a material that obviously becomes runny at about 250°C, and yet retains its shape while in the sintering furnace at 1300°C. It would be good to learn more about how this works.

A blind guess: the original filament has a resin to make the metal powder flow, that becomes viscous at certain temperatures. Perhaps this resin could be volatile and evaporate at temperatures below its viscous temperature.

Or maybe it could be quickly burned off by supplying extra oxygen (so that burning temperature isn't high and there's not enough time for the piece to deform).

In any case you could probably introduce some kind of automatic software correction for the deformation to the piece. As noted in the article this has some limitations/drawbacks, like limiting some geometries you can achieve (and requiring careful knowledge/control of your process).