Of course the diary doesn't make sense. Only Dumbledore and the Trio knew the Horcruxes were real. Even Lucius, who recognized the diary as a Dark, magical object, was unaware of its true purpose and consequences. Slughorn knew what Horcruxes were in theory, but didn't know they were hunting them for real; Regulus discovered the secret, but thought the locket was the end-all-be-all of it; Kreacher didn't give a rat's ass because elves don't bother themselves with wizarding squabbles.
*double nods*
Besides, even if someone had told Remus about the diary, it was, at that point, an inert Horcrux. Exactly what could it have done for Severus aside from trapping his soul? (But then in order to do that, Remus would have had to kill him.) That's not very Snupin.