Right, they would probably be treated as a contractor under the ABC test and the new law.

You're hung up on the "central to business" part of the test without paying attention to the nuance. Laws aren't black and white, they're many shades of grey.

If you're in the business of doing X, say programming widgets, then if you hire someone to do X (i.e, program widgets) for you, then that person will be an employee for all purposes of this law. Because they should be. They're literally doing your business activity for you, and there are plenty of legal reasons that you should be liable for their work--you're passing off their work as your own to the end customer.

OTOH, if you hire a contractor to program your back end, that guy wouldn't become an employee under the test, because you're not in the business of programming back ends. That back end may still be central to your business, but it's not your primary business activity (i.e., it's not your revenue-generating activity).

even if I closely monitor or specify how the work is to be completed.

This is the most misunderstood part of the ABC test. The ABC test doesn't require that the contractors have free reign to do whatever. It simply provides that a contractor should have free reign to accomplish the work unless the manner in which the project is completed is a relevant part of the contractual requirements. For example, specifying that a contractor program your backend in Node, with spaces instead of tabs, use git, and make use of specific AWS APIs, to match your standard coding practices in the rest of the project would be fine. Another example: telling a designer they must use Illustrator and specific design tools within Illustrator to match the processes or products of other designer work is also fine.

Monitoring work for purposes of quality assurance or quality control is also acceptable under the ABC test and is indeed even expected by the courts--not conducting QA/QC has been used as a factor indicating that the worker is an employee.