That's what a preorder is. Ordering something before the official release date (doesn't matter if they have stock in the warehouse). I believe the review embargo is on Monday, but is that also the official release date (for sale date) for the GPU?
You need to remember that most companies don't post pictures on twitter about when they have stock of an item. They just book them in and ship them when they are allowed to.
Examples
Look at threadripper. you could preorder them from the start of the week, but they were only shipped yesterday or the day before. I bet Gibbo has had stock since last week, and 8pack was probably playing with them before reviewers where even doing the unboxings.
Games in brick and mortar stores, they probably have stock a week before the official release date but they can only sell them from the date specified in the contract they signed with said publisher