Which idea? What use and what capacity? The ideas in the article run a wide gamuth of different uses and proposed capacities.

You are right if they would stuff the staircase full of stuff and a crowd to achieve the same density as a normal shop that would be probably unsafe. But that is far from the only thing which is mentioned in the article.

One plan he mentions is a sort of meeting space where people pitch him while walking up the stairs. That would be what? No extra obstruction in the staircase and a max occupancy of 5? (How many people do you realistically want to pitch simultaneously?) There is nothing unsafe about that.

The other plan he mentions is to make the stairs his private office. I don’t see the floorplans but I don’t see why you couldn’t turn the space into an office with an occupancy limit of let’s say 3. Basically an overgrown garden shed. To do that you might need to do some construction work.

The very fact that he has a list of ideas going from hard to easy means to me that he is aware of the very constraint you are mentioning.

About his more ambitious ideas: if he has sense he should sit down with an engineer who is well versed in the local safety regulations. Clearly the more obstructions you design in to the space the more limit you need to put on capacity. You can also imagine putting limits on how high people can go. You don’t need to use all of the stairs. You can for example use ground floor and the first landing as popup store, second landing as “crew only” storage space and block of the rest of the stairs. How many people can you let in the building then? Once he knows the shape of the limits he can work within he can start thinking to find a program which can safely work within those limits.

    “One idea I’m playing around with is: someone starts at the bottom of the stairs and pitches their idea to me and if, by the top of the stairs, I like it, I fund their business.”

More likely tho it's a publicity stunt for the founder networking organization he runs.