I'd be amazed if a well run magazine would run dry of material. (Especially in times of covid, because every man, woman and granny is apparently writing!) But then I'm thinking of top-tier magazines that pay like Analog. And because of this and the long-term reputation such a mag has and the relationships with lots of authors they have developed, I wouldn't be surprised if there was a gap in their output coming up, that they could not directly ask a big name if they had a story they wanted to submit, so that they could 'star' in an issue.
(There definitely has been some issues of that magazine where the number of stories was cut so that a big name author could publish a very big short story. Oh, and they also included an in-depth interview with him, so that's adding more padding!)
So one assumes if a magazine editor is desperate they would try and exploit their relationships with previous authors and other contacts, no? Most working authors who have previously submited short stories must have loads of shorts or ideas lying about.