The distinction really kinda depends on the situation.

FDEs have been around for a long time in the defense contractor space and Palantir picked up the term, broadening the meaning a decent bit. Then it spread to the rest of the software/tech space.

Historically FDEs in defense are engineers who would literally forward deploy out to other countries where the hardware was being deployed so that they could provide on the ground hardware and software support. They'd either literally be called FDEs, Engineer (forward deployed), field engineer, or some other title that roughly got the meaning across.

You'd deploy some platform and send along an engineer or two and a few technicians. Depending on the platform or the scale of the deployment the engineers would either be normal engineers forward deploying for a few months to a year or so or you'd hire a dedicated FDE for that given deployment/site.

AFAICT it became a lot less common as internet communications got better and you could do practical remote debugging and live video conferencing but you still see FDE roles in the traditional sense from time to time.

But yeah then Palantir and big tech came along and basically rendered it into a glorified consultant and/or systems integrator role.