argument or statement?

"less good" is non-standard and "maths" should be either math or mathematics. There are several ways to re-word this. For example:

Some people put forward the argument that girls are by nature not as good at math as boys.

Some people put forth the argument that girls are naturally worse than boys at math.

Sorry, I mean

Some people put forward the argument that girls are naturally less good at maths than boys.

If I replace "argument" to "statement", does it make sense too? If not, why?

Doctor D "maths" should be either math or mathematics.

Maths is the usual British contraction. Math is the American contraction.

Either "statement" or "argument" works grammatically, but they mean different things.

You state a fact or a generally accepted idea. You argue for a position which others may disagree with.

A statement: The American Civil War began with the attack upon Fort Sumter in 1861.
An argument: The American Civil War can be said to have begun with John Brown's raid in 1859.

john liaoSome people put forward the argument that girls are naturally less good at maths than boys.If I replace "argument" to "statement", does it make sense too? If not, why?

I think that 'assertion' is more apropos than 'statement'.

You could substitute "assertion" for "argument" since both indicate strong emotional involvement. But a "statement" is emotionally neutral, so neither "assertion" nor "argument" can be substituted for it.

john liao naturally less good at maths than boys.

That seems to me to reflect strong emotional involvement, not a statement (which at best would then be labelled 'ignorantly sexist').

Do not get hung up on the content of the sentence when describing the nature of the terms "argument" "assertion" "statement."
However, the "girls not as good at math as boys" can be viewed as either a statement or an argument (or an assertion). In some circles it would always be an argument (a bad one) but in other circles it would be regarded as simply an empirical statement of fact.