"but a credit added to the amount of FWH from the W-2."

"The W-2 is $414,000."

From: https://www.irs.gov/newsroom/what-is-the-additional-medicare-tax-and-who-pays-it

Example:

"Withholding/Estimated Tax. Employers must withhold this tax from your wages or compensation when they pay you more than $200,000 in a calendar year. If you are self-employed you should include this tax when you figure your estimated tax liability."

And the Tax filing uses a table...

So,in this example, they would have seen $450 overwithheld in this example, if they didn't exceed that value, because they hadn't met their threshold.