Is it correct to say "Dejemos trabajar a la fiscalía, *sino* vamos a enturbiar la investigación" or should we use "si no" instead?

Question

I thought I had the "sino" ("but rather", "with this exception", "only this thing", etc.) vs "si no" ("if not") distinction down.
Though I recently saw a confusing use of sino from a reasonably reputable source and after asking several native speakers, while they are not grammar experts nor familiar with the rules, they saw no problem with it. I reviewed the Fundéu and RAE explanations and am still at a loss.

The example is:

Dejemos trabajar a la fiscalía, sino vamos a enturbiar la investigación.

(Pic of the screen)

Especially with the use of the imperative dejemos, I cannot convince myself of any other meaning besides this: "Let us allow the prosecutor to work, if not we will muddy the investigation". With the understanding being the investigation that the prosecutor is performing, not one he is under.

Is this just a common error that native speakers (and popular news stations) make or am I missing something?

Answer 1

The use you observed is definitely incorrect.

This would have been the correct form:

Dejemos trabajar a la Fiscalía; si no, vamos a enturbiar la investigación.

The usages of sino you explained are correct; you can check the DPD article here.

And to answer your question: yes, it is a rather common error which many people make. News stations and newspapers are not an exception. In general, I'd recommend not to take any mass media as "reputable source" when it comes to grammar use.