They are starting with 25k SKUs. They have the potential to have infinite SKUs in this program, while brick and mortar stores are limited to shelf space.
That being said, it might make sense to keep the selection artificially small for multiple reasons.
Depending on how much volume you have, inventory turns are an important gate on how many items you can afford to carry. And I would imagine that there are practical limits on how many SKUs you can pull stock from and still deliver in an hour.
That being said, it might make sense to keep the selection artificially small for multiple reasons.