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For example, using "didactic" instead of using the simpler "instructive" when the use of "didactic" is not justified in the context as it could have been swapped with "instructive" without changing the meaning at all. There are some cases where it might be justified, but in our fictional example it is not. How would you describe such a writing, or such literary choice?

Normally you'd say something like "that's an unnecessarily elaborate word".

Except for verbose, none of the following are common, but I've marked the very rare:

• 
  rococo (adj) having elaborate ornamentation wiktionary (rare)


• 
  baroque (adj) very elaborate wiktionary
  


• 
  elegant variation describes using synonyms to avoid reusing a word wikipedia
  


• the lure of the abstract describes the use abstract words instead of concrete ones Plain Words (rare)


• 
  circumlocution (noun) is a speaking around the topic and being very indirect (rare)


• 
  pleonasm (noun) covers the case of using too many words wikipedia (academic, rare)


• 
  sesquipedalian (adj) is the use of long words instead of short, only ever seen humourously wiktionary (academic, rare)


• 
  logorrhea (noun, rare) is using too many words, also verbosity and prolix
  


• 
  high falutin' (adj) is a pejorative phrase for "over-educated": "You and your high-falutin' words, just trying to confuse us!" (US, very informal)

You can call this flowery language.

According to Cambridge:

flowery (adj.) disapproving If a speech or writing style is flowery, it uses too many complicated or unusual words or phrases.

Collins says:

flowery (adj.) full of figurative and ornate expressions and fine words
said of language, style, etc.

One writing coach advises:

Avoid “flowery” language at all costs! If necessary, throw out
your Thesaurus! Readers are more impressed by the quality of your ideas than your use of multi-syllabic terms.

Such a choice is a matter of tone and style. I'm not sure that I would ever agree that it could be "not justified" as there is always at least a subtle difference in rhythm and flow, and often in meaning. Intentionally choosing or avoiding words with latinate roots, for instance, can be valid style choices. Intentionally choosing a less well-known term can affect style, possibly giving an academic air, or a more everyday feel to a piece of prose. Also, a word's history, via its etymology, can influence how it will affect knowledgeable readers.

So I might describe such a choice as "using a more intricate style" or "a more complex style". Style should suit purpose, of course. If the intended audience will not be likely to get a nuance, and may well misunderstand a word, that was a stylistically poor choice. If the intended effect will not be enhanced by a particular choice, that is also poor style.