"Biden did declare it a federal emergency disaster under the Stafford Act"

You have to read the Dates of applicability for the specific disaster declaration and type.

The IRS doesn't need to issue a bunch of specific guidance, typically. They have Disaster Guidance already. Since they are not in charge of Disasters, they have relative guidance that will apply. FEMA controls Specific guidance.

It would probably be more help to you, if you don't find what you want from FEMA, IRS, and other resources, to specifically ask your detailed question on the forum. Otherwise, you will get general guidance. You would need to read and look up things for your client, anyway, of course.

Because what Applies and who Qualifies are not the same for every given disaster. Someone on the forum asked about taking Fraud as a disaster loss, simply because it fell in the timeframe of coronavirus, but in that example, it is not attributable to the disaster declaration or virus impacts.