Use of article with literary devices

I am writing encyclopedia-like texts about literary devices and am wondering if I should use the names of literary devices (e.g. allegory, oxymoron, dichotomy) with or without the indefinite article "a/an". Does it depend on the context? Here's an example from Wikipedia:

"As a https://en.wikipedia.org/wiki/Literary_device, an allegory is a https://en.wikipedia.org/wiki/Metaphor whose vehicle may be a character, place or event, representing real-world issues and occurrences. Allegory (in the sense of the practice and use of allegorical devices and works) has occurred widely throughout history in all forms of https://en.wikipedia.org/wiki/Art..."

The word "allegory" here is used with the article in the first sentence and without it in the second sentence. Is there a mistake or is it the way it should be used? Why? Help me clear it up, please.