You cannot retrospectively prove inclusive sourcing if the algorithm screened the pool first.
Posting on Ethnic Jobsite helps create a dated, vacancy level record showing your organisation took active steps to reach candidates from groups that algorithmic tools may disadvantage. That record exists before shortlisting begins.
Under the Equality Act 2010 and, for public bodies, the Public Sector Equality Duty, employers may need to show how they supported equality of opportunity in practice. A specialist channel placement gives them clearer, role specific evidence that targeted outreach took place before shortlisting.