Uhm, no?
There is no such a rule or benefit anywhere regarding this. I don't know who ever told you this but is total a lie.
ANYONE can hire an artist, that means you could also open a thread in the Collaboration category and a "Hey I need a cover, I will pay an artist to make one". The author searches for an artist (here or in social media) and when they find one, they order the commission, they pay them and voila, they get the drawing.
At the same time, there is either people who get someone to draw for them for free, be it because they are friends or they luckily found someone doing limited free art requests.
You also have people who draw by themselves and do their own artwork and content.
Then you also have those that open Google and just pick an image from the internet and use it as a cover (not recommended, may violate someone's intellectual property or copyright)
Sub amount is no requirement, you can literally do all the prep and have the image ready before publishing, after all when you load a file for thumbnail/cover is from within the files in your computer, not an exclusive database or cloud within the server of Tapas.