my retirement account was hacked through my iPhone

I agree with Kurt Lang, you were given false information about your iPhone. There are several ways this can happen and none of them involve a virus on your phone:

• Message claims someone else logged in and when you click the button to confirm/deny, it asks for your credentials. You have just given the scammer access to your account.
• Message from someone claiming to be Apple and you gave them your Apple ID and Password.
• Your account may have suffered a data breach and instead of taking responsibility, they are blaming you. They are required by law to report it, but there may be a delay while they evaluate the impact and will not disclose any information until an official statement is given by the company.

The solutions are the same, change your Retirement Account Password and your Apple ID Password. To prevent it from happening again, review this support article to recognize and avoid phishing attempts.

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

A message cannot hack anything. It's just text that says whatever they want it to say. The buttons don't mean anything, either.

Their fraud division is apparently staffed by idiots. iPhones cannot get viruses, or any other type of malware. The only current threats are hacks that cost 1 million dollars or more to try and gain direct access to ONE device. Is your data or retirement benefits worth that much to someone?

The Geek Squad will only succeed in royally screwing up your phone. Do not let them touch it. You could bring it in to an Apple Store, but I can already tell you there will be nothing for them to find.

And who did you contact about a sign in attempt?

Regardless, there are only two things you should need to do. First, change the password for the Apple ID your phone is using. If a crook has access to your account, it's not from your phone, but by guessing, or finding out what your account name and password is. With that information, they can login to your account as "you" from any other device. Second is to change the retirement fund account's password.