"\dE" displays the list with a "List of relations" heading whereas "\det" displays "List of foreign tables". So, to differentiate the two, I suggest to change the help message for "\dE" to:
\dE[S+] [PATTERN] list foreign relations
One could argue that both essentially mean the same thing, however, considering "\dE+" also outputs size, it makes sense IMHO to make this change (as PG documentation: relation is essentially a mathematical term for table). Attached is the patch that makes this change.
help.c and the documentation need to be synchronized a bit more than this single issue.
Calling it "foreign relation" for \dE and "foreign table" for \det does convey that there is a difference - not sure it a huge improvement though. The "\d[Eimstv]" family of meta-commands should, in the help, probably be moved together to show the fact that they are basically "list relation names [of this type only]" while "\det" is "list foreign table info".