That is exactly what they mean. As it’s invite only, they are offering limited “spots” for sellers so they don’t want anyone who is planning on selling something that they don’t actually have available for sale now.
I applied and didn’t get invited, so it’s not as easy as just completing their requirements. From what I understand, they are only looking for a few sellers in each category as they don’t want to deal with the drama that comes with competitor games like we see on Amazon. Smart decision in my view. They said they would open it up more in the future so there may be opportunities later.