Only by having the source code can you confirm it is E2EE, which we cant because we are not the Whatsapp dev.
Also even if it is E2EE, if the implementation is borked, it is good as unencrypted. Who has access to the actual encryption keys? If you dont have access to the key or if the admins have access to it, then another person can decrypt and access the messages in it.
Maybe a 3rd party audit can vouch for it but the findings will be buried in legal-speak which needs knowledge, attention to detail and reading between the lines in order to uncover bad designs. Also, after the actual audit, nothing stops FB from altering code to whatever they want.
A good way to test is to file a case against your (or another) account and have the courts subpoena an encrypted message. If you can see the message in court, it is not true/proper E2EE. However, there may also be laws which prevent a case involving just Joe Average from spilling decryption capabilities if it is in the US interest to keep decryption capabilities a secret.