Out of the 3 businesses they name, 2 are abuse based businesses. Park.io is for-hire domain-swiping, closet tools is follower and social network automation and spamming tool ...

Best way to start on the path to building one of these businesses is to start building a following. Many of these 1 man companies are reliant on strong founder personalities that develop a following. Makes distribution way easier.

These are solo founder businesses. Many of the examples hired employees. If you have a boring b2b SaaS that charges $100 a month, you only need 1000 monthly customers to gross $1M a year. You don't need a personality, you need a decent funnel.

Assuming B2B. Can one individual manage the demands of 1000 clients?

No. If you look at the article however, many of the examples have employees.

This seems absolutely important