Why does 'first' make a difference to the views of the person?
I sometimes think that you don't even bother to read what I post.
I made the point (using Thatcher as an example) that the notion of being a trailblazer is significant even if you may not agree with the views of that trailblazer (as was the case for me with Thatcher).
How hard is it for you to understand that if there has never been an ethnic minority PM that for people from ethnic minorities to think that being PM is something they can never aspire to as regardless of their views. Effectively that their non majority ethnicity will be a deal-breaker preventing them from being elected. The
first ethnic minority PM (or woman PM, or muslim PM etc, etc) breaks that glass ceiling. It demonstrates that being from an ethnic minority (or being a woman, or muslim) is no fundamental barrier ... but your views may be if they are not attractive enough to the electorate.
That's exactly what the article does - it detaches the notion of identity (black, muslim, female etc, etc) which is no longer a block to being PM etc from views, which may well still be a block unless they are sufficiently attractive to the electorate.
Are you really arguing that the election of Thatcher was not significant for the ability of women in the UK to consider that they could become PM, or that the election of election of Obama was not significant for the ability of black people in the US to consider that they could become President, regardless of whether or not you agreed with the views of Thatcher or Obama.