Where did you see they are available for US citizens? This is “Where is Revolut available” from the FAQs:
Revolut is available to legal residents of the following countries:
Aland Islands, Austria, Azores, Belgium, Bulgaria, Canary Islands, Channel Islands, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, French Guiana, Germany, Gibraltar, Greece, Guadeloupe, Hungary, Iceland, Ireland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Martinique, Mayotte, Netherlands, Norway, Poland, Portugal, Réunion, Romania, Saint Helena, Slovakia, Slovenia, Spain, St. Martin, Sweden, Switzerland and United Kingdom.
In the future we aim to become available worldwide!