Describing the reparations demand as a political demand rather than a financial instrument is not a dismissal of the underlying moral concerns. It is a description of what the demand actually is.
Political demands can be legitimate and important even when they cannot be converted directly into legal or financial obligations. The demand that Britain acknowledge and honestly reckon with its history of slavery is a legitimate political demand. The demand that Britain improve trade terms with Caribbean nations is a legitimate policy proposal. The demand that a payment be made to foreign governments from a racially discriminatory levy is neither of these things. It is a demand that cannot be met without creating new injustices, and that is why its advocates have never seriously engaged with how it would work.