True. But the oldest galaxies (what Herschel is mainly designed to look at) don't emit in the visible, even in their own (rest) frame. That's because the earliest galaxies are very dusty, and all this dust is opaque to the visible light. The stars are still there, glowing away, but their light is absorbed by this dust. This absorption heats the dust, warming it to 35K (give or take), which, as all things with non-zero temperature do, emits radiation like a blackbody. This light is then redshifted such that it's blackbody spectrum peaks in the submillimeter, which is what Herschel looks at.
Disclaimer: I work on BLAST, a balloon-borne experiment (cheaper than a satellite) which has detectors nearly identical to the ones of the SPIRE (main) instrument on Herschel.