You gave the IRS definition.

How do you know if you should be called an employee or an independent contractor?  For a quick guideline, let's go to the IRS.  They say that by-and-large you are an employee.

However, it does not apply under all legal regimes.

For example, the FLSA definition of who is in an employee-employer relationship is if the employer "suffers or permits" the person to work. No saying: Gosh I never told that person to work on the line.

The NLRA's is very broad, with the presumption being for employee. It applies outside the employment relationship so that employees can engage in acts of solidarity with one another. It says:

The term "employee" shall include any employee, and shall not be limited to the employees of a particular employer, unless the Act explicitly states otherwise . . ..

For more on this issue, read this article:

Not a Limited, Confined, or Private Matter: Who is an Employee Under the National Labor Relations Act

Abstract:    

Congress defined employee under the National Labor Relations Act to say that an employee is any employee and is not limited to employees of a single employer. It was the intent of Congress that worker rights and labor organizing was to extend beyond the scope of a single employer, because only in this way could the bargaining power of employers and employees be equalized. Only protections based on such a broad definition could permit employees to overcome the inequality of bargaining power that was created by corporation law.

The courts, however, have not only not enforced the plain meaning of the statute nor the intent of Congress, they have gone so far in some cases as to define employee as limited to the employees of a single employer. The common law system of judicial decision-making means that judges continue to veer farther away from the rights Congress intended to provide employees. This article explores that legislative history and current cases pending decision in which the question of the definition of employee is critical to the outcome.