Why not
Axiom's terms say current or former users do not count as new users. So a code only applies when a new person creates a new account.
Should I make a second account to use a code?
No. Axiom's terms (section 14) do not allow self-referral or multiple accounts.
What still applies to you
Axiom's own fee ranks run from Wood to Champion and pay part of your fees back in SOL as you trade more. That is Axiom's own program, not part of this guide.
New to Axiom?
Then the code applies to you: use the referral link when you sign up.