The [selectors] spec section linked from that paragraph contains the detailed algorithm of specificity calculation, including the following paragraph:

If the selector is a selector list, this number is calculated for each selector in the list. For a given matching process against the list, the specificity in effect is that of the most specific selector in the list that matches.

It doesn't strictly declare the equivalence between "the specificity in effect" and "the specificity of the rule" (though strongly implies it), but seems quite unobvious which specificity value will be in effect for any given selector, and therefore for the rule associated with it. IMO making the text too verbose while providing the link to the detailed algorithm would be not very useful, however a minor editorial change to remove the rest of ambiguity from it could be helpful.

The [selectors] spec section linked from that paragraph contains the detailed algorithm of specificity calculation

This links to https://www.w3.org/TR/selectors/#specificity which explains how to calculate specificity.

Oh wait, reopening as I didn’t see your suggestion at the end there.

I think the text should say something like "Each declaration has the same specificity as the highest specificity matching selector belonging to the rule it appears in."

Technically even better is to say that declarations have no specificity, because it’s the selector that determines that.

Maybe the text like this would be better than existing one?

The Selectors module [SELECT] describes how to compute the specificity of a selector. The declaration from the style rule with the highest specificity of the selector wins.

The Selectors module [SELECT] describes how to compute the specificity of a selector. The declaration from the style rule with the highest specificity of the selector wins.

That works for me. Getting rid of the notion of declarations and rules having specificity seems the neatest approach.