Here are a few telltale phrases that will help editors notice when they're describing their own impression of the show instead of describing the show itself.
- it is shown - is it really shown? If it were really shown, you should describe the event in the active voice. Describe who's doing it, what is being done, and how it's being done. Sometimes a character tells of an event and it is never actually shown, like unicorns raising the sun and moon in Hearth's Warming Eve. The description of the episode should say that the Spike tells that the unicorns raised the sun and moon, and not that "it is shown". Other times the phrase "it is shown" simply masks speculation.
- it is unclear / it is not explained / it's not shown / it is unknown - this is very often a preamble to speculation, since it inevitably covers material that is not presented outright in the show. The best thing to do is not describe things that are not described in the show, to avoid filling the gaps. For example stating that Rarity did not go to Celestia's School of Gifted Unicorns may be correct, but it is never presented outright in the show.
- in fact / caused by / can only be because - if it's a fact, cite it against the specific event from the show, and don't infer generalizations unless they are presented outright in the show.
The easiest way to avoid in-universe descriptions and speculation is to cite everything you write against specific instances in the show. That way both you and the reader can be certain that your description actually matches what is depicted in the show. When writing episode summaries, the easiest way to avoid in-universe descriptions is to focus on the cinematic elements of the show, instead of trying to re-tell the story of the episode.