No, you've missed the point, to be Dark Jedi you have to be a member/former member of the Jedi order, this is part of the star wars canon. Dark jedi have the benefits of techniques from both sides of the force, making them much more dangerous than other darksiders.
Some times non-force users describe Darksiders as Dark Jedi, this is akin to describing someone with a gun as a soldier, understandable but not something a professional would do.
Dark Jedi is what we use to call all Dark Siders, sure most of them are either Sith or former Jedi, but that does not mean they must be a former Jedi.
"A dark Jedi is anyone who practices the dark side of the Force whether or not they were a former Jedi."