When I was getting recruiter inbound for this role at Palantir >15 years ago (I think!), it was basically a implementation role where you're airdropped into somewhere secret squirrelly to write software that does secret squirrel things. Basically the same thing as what defense contractors or many consulting companies were already doing (this was basically my life at ThoughtWorks, but private sector) but with more tech-company marketing.

Now that the term is back en-vogue, FDEs can either be this or a pre-sales solutions engineer who will also implement the thing with the customer before handing it off to professional services who will take it to production. AFAIK it's still very software engineering heavy.