It explicitly calls out Kratom in their terms:

"Sale of a legal substance that provides the same effect as an illegal drug (e.g., salvia, kratom)"

If you argue against that definition, they provide a general catch-all for products that have are deemed illegal in certain jurisdictions:

"...products and services with varying legal status on a state-by-state basis; goods or services, the sale of which is illegal under applicable law in the jurisdictions to which your business is targeted or directed"

Kratom is illegal in several US states and in several countries:

https://www.sprouthealthgroup.com/substances/is-kratom-legal...

I know you're clinging to the words "controlled substance", but the truth is Kratom violates Stripe's terms in multiple ways.