"But the first question was about SELF EMPLOYMENT TAX, and that question was NEVER answered."

Yes, it's been specifically stated: it's business income the same as if a customer paid you. There is nothing different about it. The grant was given in place of business income, so the Feds treat it as business income. If your taxpayer is Sched C, this is subject to SE, because it is business income.

Unless there is a specific exclusion for that grant (such as Shuttered Venues and Food Banks) and unless your State treats it differently under some exclusion, all you do is consider this part of Gross Revenue for the business.