A mathematician is someone who creates or advances math. Not someone who uses math. If you don't understand how the word is used, that's your problem, not a problem with the statement.
mathematician /măth″ə-mə-tĭsh′ən/
A person skilled or learned in mathematics.
One versed in mathematics.
An expert on mathematics.
The American Heritage® Dictionary of the English Language, 5th Edition • More at Wordnik
None of the definitions require creating or advancing mathematics.
People in the field would never, ever call someone "versed in mathematics" as being a mathematician.
I don't even buy that regular people call someone who is "versed in mathematics" a "mathematician".
If you met a 30 year old person and asked what they do and they said "I'm an athlete" - do you take the dictionary definition there too? Most people will assume they mean a professional athlete.