What do you call someone who is very careful with their word choice?
Solution 1:
How about lexicographer? From Lexico:
lexicographer: A person who compiles dictionaries.
Your example:
Only a lexicographer would know the meaning of X.
Another possibility is vocabularian. From Wiktionary:
vocabularian: One who cultivates a large vocabulary.
Your example:
Only a vocabularian would know the meaning of X.