There's a mismatch between the rule applicability, the rule example wording and the HTML spec:

• Rule applicability allows headers to reference td or th elements (cells, as defined by the HTML Standard)
• Rule wording in the examples suggest role=rowheader or role=columnheader is cause of pass/fail, but these aren't relevant to the rule applicability
• The HTML Standard only allows headers to reference th elements

Rule Applicability

Each target's attribute value is a set of space separated tokens. Each token is the value of the id attribute of an element, that is a cell of the same table.

https://act-rules.github.io/rules/a25f45#expectation-1

HTML Standard

The HTML living standard requires headers to reference th elements

The headers attribute, if specified, must contain a string consisting of an unordered set of unique space-separated tokens, none of which are identical to another token and each of which must have the value of an ID of a th element taking part in the same table as the td or th element (as defined by the table model).

Rule Examples

The rule examples suggest that role=rowheader or role=columnheader is relevant to passing or failing the rule:

https://act-rules.github.io/rules/a25f45#passed-example-5
https://act-rules.github.io/rules/a25f45#failed-example-4