i dont know about musk and cook.
however saying "liquidmetal is not a good metal for large car parts" is a bit of a stretch. at this time i would think could be used in medium sized strips, foam or connectors to reinforce a car structure beneath the outer shell. (as the method for making large sheets becomes perfected they can be incorporated into the future designs.) believe me automotive engineers love to solve this kind of problem. also modular design can allow medium size parts to be replaced in units that interconnect.
there are lots of parts in a car that do not get hot enough to effect liquidmetal. buttons, dashboard, linkage, cables, instruments, transmission, engine parts that are not in combustion area (such as carburetors), wheel rims, axles, nuts and bolts, compressors, air bag triggers, etc. etc.
there is not enough space on this page for the parts that liquid metal can make better.
$^)