We only have your report of what you asked them and what they answered.
When you said “everything which is written in the bass clef…” that is just wrong. Most 20th/21st century scores don’t follow that convention.
The statement that “everything is transposed from C to F” is also just wrong. Composers as late as Brahms and Wagner wrote using the notation for natural horns, not Horns in F. Wagner sometimes wrote horns parts in 6 or 7 different transpositions at different times in a single score, to indicate that a valve horn player was supposed to use the same harmonics as the corresponding natural horn would have used, not “modern” (and easier to play) fingering.
Professional horn players should be able to read whatever transposition and notation convention is put in front of them.
In fact there are a few notorious instances where it is not at clear which bass clef notation the composer actually intended, since the music can be played either way and neither option “sounds obviously wrong”. (There are similar problems with some 18th century works using natural horns in Bb or C, which were made in two versions sounding an octave apart).