No, there was no leaked database of 3.5B #WhatsApp users. Researchers from Vienna simply used the contact discovery method with all possible phone numbers to get contact data of 3.5B users. They informed WhatsApp who kinda fixed it by setting rate limits for contact discovery and the researchers deleted their collected data. Read the paper instead of clickbait :)
https://github.com/sbaresearch/whatsapp-census/blob/main/Hey_there_You_are_using_WhatsApp.pdf