Age verification requires a document that can be matched to your ID, such as by the photo on your ID card.

Credit cards don't have photos.

> How many Americans wouldn't be able to present a CC or ID?

The number of Americans who don't have a government issued photo ID is estimated around 1%. The number gets larger if you start going by technicalities like having an expired ID that hasn't been renewed yet.

The intersection between the 1% of 18+ Americans who don't have an ID and those who want to fully verify their Discord accounts is probably a very small number.