Something explicitly shown in the movie doesn't become a "fan theory" just because the movie doesn't hold the viewer's hand and spell it out for us, or requires more than one viewing to put all of the pieces together.

You're right, no one actually says "Wow, Snape took those guys out on purpose." But we're talking about a master wizard, a good guy pretending to be a bad guy, being attacked and deflecting that attack to take out the only other bad guys present, which both helps out the good guys and keeps his cover intact.

What's more likely, that he did it on purpose? Or that he got lucky? Logic dictates that it appeared to be an accident, and the reveal of his actual allegiance suggests it was on purpose. Everything we know suggests it was meant to be something you realized after the fact. This isn't a "fan theory" so much as "rudimentary observation of the film."

Like, Obi-Wan's talk with Luke about his father and Vader in A New Hope is seen in a new light once we learn the truth in Empire Strikes Back. That doesn't make "Obi-Wan knew the truth about Vader and lied to Luke" a fan theory. It's just a thing that happened.