You can definitely post a short story as a single novel! But yes, Tapas analytics does prefer series that continuously updates. Every time you update, it'll appear in the Fresh section of the website/app, so that's where you can gain more visibility. So I agree that a collection of short stories would probably do better if they're under a particular theme or concept. 
There are some examples of those collections on Tapas:
My own scifi collection:
@wh_vigo 's horror collection:
Or spin-offs an existing novel with loose connections to the original novel:
@CodeMonkeyArts puts the characters from her novel into different AUs.
@sarahmassey998 has multiple stories within the same horror universe