Self-employment is a kind of legal working status. Think of an handyman (or an Uber driver), they might have a company or be self-employed.

Since having a company has much more legal work and expenses, many workers prefer to be self-employed. And usually self-employment seems to only happen in lower income jobs.

In this case bogus means that the self-employed status is being abused by a company to reduce tax payment and associated costs and benefits that an employee would have.

That someone is on paper self-employed but actually performing work as an employee would, and thus the employer should pay employment taxes, insurance, ... for them.