What's the term for contradictory sentences or phrases?

Welcome! Well asked. I'm sorry you're having trouble with this.

The statement is not contradictory or paradoxical.

"Don't include too much technical detail if it doesn't add value"

This is two ideas together joined by the if. It requests the listener not to include excess technical detail, usually meaning jargon that is confusing to the uninitiated. The second idea is the reason for the initial request.

"Because technical detail (not understood) does not add value to the explanation.

Meaning that it does not contribute to complete understanding.

If the phrase seems contradictory do what you can to pull it apart into its individual ideas. Speech and casual writing makes such ideas run together without regard to those who will later read what people say. Your style guide may do well to ask that sentences are put in their simplest and most direct form. I cannot recommend such guides, only good practices.

Perhaps your rule should be "Avoid using unnecessary words" or "Don't use any more words than necessary" or "Don't use superfluous words."

Why? Because your example sentence -- "Don't include too much technical detail if it doesn't add value." -- uses unnecessary/superfluous words. Instead, you could say one of the following:

"Don't include too much technical detail." --> Here I have eliminated "if it doesn't add value", which is superfluous given you use of "too much."

"Don't include technical detail that doesn't add value." --> Here I have eliminated ""too much", which is superfluous given your use of "that doesn't and value."

From Lexico:

superfluous: Unnecessary, especially through being more than enough.

Doubling negatives is frowned upon in guidelines for Plain Engish: "Use extra technical detail only if it adds value." https://www.plainlanguage.gov/guidelines/concise/use-positive-language/

I work in science, so I understand the need to make text accessible to a wide audience, but it's unclear from your initial question if this is the point of your post or if you are looking for the concept rather than a specific sentence.

If the prior, it seems to me that your issues include:

1. avoiding jargon because it's hard for the general public to understand (as @Elliot mentioned above)
2. avoiding wordiness/verbosity in your style guide

Here's an idea: "Avoid unnecessary jargon because it alienates casual readers."

The "unnecessary" encompasses the idea that some jargon might be necessary, but they are cautioned against using it excessively. "Jargon" means words that are specific to an "in crowd" or the uninitiated (as @Elliot mentioned also), so that encompasses the type of words. You can play around with the audience word(s). I like "casual readers" or "laypeople/laypersons."

I also love the word "granular" when I'm dealing with excessive technical details.

EG: "Avoid granular explanations."

I work in a scientific "silo" and I still avoid being granular! [insert laughs]