well, i agree and disagree.
First off the Sith is more then an order (seeing as it only ever consists of 2) so it has teachings, teachings that anakin would not have been taught.
however he is not a dark jedi, he is a sith, he accepted palpatine as his master and accepted the name Darth Vader, so he was a Sith.
but technically he knew nothing of the Sith teachings, so he basically was just using his jedi training for evil.
although palpatine had been giving him little bits of sith teachings, i.e trust your feelings...anger is not a bad things .etc. etc. for the whole of his life.
but yes, he was a Sith, but a Sith of the lowest level, kinda.
the thing is that he already knew all about the force, he just needed to be taught all the Sith stuff
"use your aggressive feelings boy! let the hate flow through you!"
all that kinda stuff
to be a sith is simply to use the jedi techings and spiking
it with raw emotion. "it was trained in the jedi arts" quigon said.
palpatine alluded to special sith training, but he's also a liar.
once anakin turned to the darkside, and accepted palpatine as his master,
he was a sith in every sense of the word. simple as that.
edit: oops...just realised ((The_Anomaly)) got to it before me...oh well 😛