That would come under "unfounded assumptions of higher productivity", then.
If the worker isn't getting sick pay (etc.), then they're not really getting more money because they have to accrue against all those eventualities (which is more efficiently managed at aggregate level by a business, rather than individual level). It may feel like more, but that's a dangerous illusion.
Training and equipment? If the worker needs those to do the job, then they need it regardless. If they don't, then we're not talking about the same worker.
As for line management - that's another way of saying that self-employed people have to do a bunch of unpaid overhead work for themselves. So, basically it's shifting time costs from the employer to the employee. Again, the "more" money to the employee is an illusion. It also assumes that all managers (including both line managers and accountants) spend more time on managing their direct reports than they do dealing with contractors - that may be true, but it's far from self-evident. Someone has to check all those invoices and timesheets...