That is incorrect.
They don't even have an anthology call out at the moment, so I think you're conflating their short-story anthologies with their novella collections. The call for 20,000- to 60,000-word stories is a collection of novellas, sold separately and for which LT3's standard royalty system applies. Here's the current novella collection call:
https://www.lessthanthreepress.com/collection-calls/
Here's a Wayback Machine snapshot of their last short-story anthology call. The short stories are minimum 10,000-words, maximum 20,000-words, for a flat-fee of $200.
https://web.archive.org/web/20170102001835/https://www.lessthanthreepress.com/anthology-submissions/
My story (not in this particular anthology volume) came in at 10,500-words.
I wish you the best of luck with your Ninestar story. I'll be interested to hear what kind of sales numbers the anthology pulls down, if you are willing to share that information when the time comes.
ETA: I just looked up Ninestar's current anthology call (
Teacher's Pet), which asks for stories in the 7,000- to 30,000-word range (but will also consider other lengths). Their
submissions FAQ states that for multi-author anthologies, they "pay each author an equal share of 50% of the download prices from our website and 50% of the net royalties from third-party vendors."
Sticking to the stated range, that means an author who submits a 30,000-word story will receive the same royalty as an author who submits a 7,000-word story. Most anthologies I've submitted to that had such a wide word-count range either paid escalating flat-fees for sub-ranges within the overall range, or they paid royalties to the contributing authors based on each story's percentage of the volume's total word count.