Maybe you mean where in bill does it say a US photo ID is required.
I think your are correct, I don't believe such a requirement exists in the bill itself and that's a big part of the article. Because the law doesn't require this, there's a very real risk that people won't realize that, if you are an adult, you will be required to upload a photo of your state ID. Prople might support the bill and only realize what it means in practical terms once it is too late and it has become law.
It isn't clear to me if requiring photo ID is a practical requirement or a decision the two incumbent vendors have made for their own reasons. My guess is that it is the cheapest and easiest solution.
There is no other technology to do age verification at scale.
Apple, Google and such will contract out this age verification to a third-party which will ask you to upload your ID and a 3D face captcha, which the third party will delete within 15 days, but somehow magically still make it into an unfortunate, unavoidable data leak a couple of years later.
I think you are assuming what their definition of "verify" is going to be, but it's not actually written in the text of the bill, so we don't know. Similar laws in some states only asked the OS to collect the age, it specifically doesn't say that the information must ever be accurate, stored or used for anything.
Collecting the age will be done via a photo of a legal US state ID. We can take bets but, as the article points out, only two vendors can do this and this is how they do it.