Despite knowing that Snape is a brilliant Potions Master, was in the same year as him, and (presumably) having friends who used spells from Snape's book, Remus does not seem to draw any possible connection to Snape's being the Halfblood Prince at Christmas.

Remus is, and has been, a ungrateful, self- centered liar.

If that is too harsh, than perhaps economical with the truth is better.