This is not a pre-order early access to beta as you state, that was last weekend. This weekend is open beta, so it´s open to everyone. Dont have the exact numbers but last weekend there where over a million players. This weekend anyone can download and play, even people that have not pre purchased so there will be ALLOT more of people. Blizzard made a special post where they did warn about this.
https://eu.forums.blizzard.com/en/d4/t/expectations-for-the-diablo-iv-open-beta-march-24-26/978