IMHO, they've wildly missed the mark. APIs, as colloquially known, can't be RESTful as they define it because the client systems aren't AI based and can't follow links. To use their example:

  <html>
      <body>
          <div>Account number: 12345</div>
          <div>Balance: $100.00 USD</div>
          <div>Links:
            <a href="/accounts/12345/deposits">deposits</a>
            <a href="/accounts/12345/withdrawals">withdrawals</a>
            <a href="/accounts/12345/transfers">transfers</a>
            <a href="/accounts/12345/close-requests">close-requests</a>
        </div>
    <body>
  </html>

I can navigate to that page and because I know English can follow links to my withdrawls and deposits. A computer can't. The client program needs to have an understanding of withdrawl and deposit in order to function. The only way to do that involves coupling the client to the server.