I'm not familiar with Dvorak's Humoresque, but assuming he wanted a section in the parallel minor (i.e. G♭ minor), then a 9-flat key signature would certainly be awkward. F♯ minor is certainly warranted here.
A number of years ago I read a review of Gustav Mahler's 9th Symphony, where the reviewer mentioned the first movement "...alternated between D major/minor..." and the finale "...D♭ major/minor...", so I can't see anything wrong with describing works in an enharmonic context when convenient. For the record, Mahler uses C♯ minor for the parallel minor sections of his 9th's Finale.
While we're on the topic of F♯/G♭, and Mahler, notice how he does this very modulation in his 10th Symphony. Just look at all those accidentals!
(Never mind that Mahler scores a fortissimo orchestral tutti shortly afterward using nine of the 12 tones on the chromatic scale, simultaneously!) :-O