Visuals don't necessarily mean psychedelia, in the end this is a matter of semantics but for psychedelia there needs to be some level of mind expansion. Generally things like deliriums, hypnogogia / hypnopompia, opiate induced revery or GHB visuals are examples of things that are not considered psychedelic although they share some aspects with hallmark psychedelia like visuals which are strongly associated with psychedelic trips.
Cannabis is a quite special drug in that it has properties that are seen in psychedelics, dissociatives, stimulants, depressants, analgesics and maybe more classes of drugs. So no argument there.
Codeine or other opiates can produce visuals or dreamy thoughts on their own but in the end they are mind narrowing.
Combining it with cannabis produces a cocktail of effects that may or may not be classified as psychedelic: with enough blunting of the codeine, psychedelia from cannabis can disappear but with strong enough cannabis effects there can be some definite trippiness.
So who's to decide?
In any case, I just thought I'd provide you with some guidelines on the definitions.