from The Century Dictionary and Cyclopedia
- n. A treasurer.
Sorry, no etymologies found.
So clearly a thesaurer is not merely a lexicographer.
A thesaurer is clearly defined as someone who lords over a thesaurus (the second definition), so it’s not great leap to extend its definition to include one who lords over a thesaurus (the first definition).
And, in case you were wondering, thesaurer (meaning “one who makes thesauruses”) is of questionable derivation; it is actually an archaic form of treasurer.
Of course, your objection would be ill-founded, because the lexicographer in me would balk at the thought of putting two identical words in his dictionary, whereas the thesaurer in me would revel in the opportunity to add more words to his thesauri (and that is the proper plural form, according to the OED).
Thankfully for the thesaurer in me, it’s both (or at least that’s what the OED says).