In the screenwriting realm, a script reader also has this habitus - it's their job to say what is good and bad. Take someone like Lucy - she spends day after day reading through scripts, widening her knowledge of what makes a great story. So you'd assume that Lucy's scripts are going to be of a very high standard.
This isn't always the case, of course. Sometimes a critic or reader might be able to spot the flaws in another's script, but be completely incapable of writing anything half decent themselves.
But what is essential here is that they have habitus - that natural knowledge of what a good screenplay is; they are already in the world and have a feel for the game. And from there, they are seen as having a certain degree of cultural capital or prestige. We assume their ability to write, based on their knowledge.
On a smaller scale, if I were to say to you - "my favourite movie of all time is Transformers 2: Revenge of the Fallen," what would you think? Do you think my script would be better than the guy who says his favourite film is The Shawshank Redemption? Unlikely.
This is why a lot of writers in the late Victorian era established themselves as critics before turning to fiction. People like Dickens, Wilde et al. If you show you know what you're talking about, you carry with you a high level of expectation of your writing ability.
So if you're stuck for where to go in your writing career, it might be worth looking into some script reading courses. You might not want to be a script reader but by going on that course and establishing yourself in that world, you gain habitus and prestige - the expectation that what you produce will be good. And you might actually become good in the process!