Indistinguishable Fiction

The other day, I got one of the most curious rejection letters I've ever seen. This was from a BIG magazine that publishes one story per month in addition to a lot of other content.

In their rejection, they said they want stories that are indistinguishable from the other content in the magazine.

Indistinguishable?

adjective
Not distinguishable, especially:

Impossible to differentiate or tell apart: indistinguishable twins; markings
that make a moth indistinguishable from its background.
Impossible to discern; imperceptible: a sound that was indistinguishable to the human ear.

Difficult to understand or make out; vague: indistinguishable speech.

In art, idistinguishable indicates something that is merely decorative. Motel room paintings. Elevator music.

Since when is indistinguishable a desirable trait for fiction? Is one of the most prestigious markets for short fiction actually looking for stories that blend in with the background?

I'm at a Losfer Words (Big 'Orra).