This should have been set immediately after ROTS
Everyone complains - and rightly so - about this being non canon as it's going to contradict the ANH "Now I am the master" line.
However, Obi Wan does mention to Luke in one of the OT sequels (probably ROTJ) that he too thought Vader could be turned at one point but that he was wrong. So there was some scope to have this set immediately post ROTS with Obi-Wan making an attempt to reach out to Anakin once he'd discovered he'd survived...
Having said that, Anakin clearly isn't just a "learner" by the time of ROTS so I guess it is questionable whether you can even describe that film as within the OT continuity canon.