Except it doesn't work as you describe. The first time, it said there were 48 in stock. Added one to my cart, and a pop up said they were out of stock.
So I Went back and looked at the item again. It STILL said 48 in stock.
Same thing the second time, but fewer were in stock. Less than 30. Got it into my cart, then proceeded to checkout, filled out the form, clicked submit, and THEN it said out of stock. Look at the item, sure as shit, still a bunch listed in stock.
I don't care what you say, or how you try to explain it away, their website is a piece of shit.
Your issue is that WooCommerce doesn't subtract stock counts on the frontend from pending orders. If they have a bunch of orders that are in pending status, which is the initial status they go into, it will still show stock as being available until those orders change to processing. That stock isn't necessarily available though (it's pending), which is why it complains after trying to add to cart. Again, WooCommerce order status doesn't change from pending to processing until payment is confirmed and then stock is reduced. They had information posted previously that stated they were clearing unpaid pending orders every 80 minutes.
If you're this pissed cause you're not quick enough, imagine how pissed you'd be if there was no stock check in place for pending orders, they let your order go through, and then they send you an email days later saying it was cancelled because someone paid before you and there's no stock left. Not only do you not get your order, but you would most likely lose whatever BTC you paid in tx fees. It works the way it does for a reason.
If you understand WooCommerce order statuses, and it's saying there's stock available, letting you add to cart, but not letting you checkout, then you'd know that stock is pending and that you should leave it in your cart and keep trying to checkout as it may become available once pending orders are cleared. When it's completely out of stock, you can't even add to your cart. Reducing stock for pending orders would most likely result in you never getting a chance to add to cart and always showing out of stock immediately.
If you don't like it, simple solution is to use an extension or write a script that posts the form data automatically and non-stop until successful. Chances are multiple people are already doing that and that's why it's difficult to order.