An ETN doesn't have to be backed by anything other than the issuer.

https://en.wikipedia.org/wiki/Exchange-traded_note

'An exchange-traded note (ETN) is a senior, unsecured, unsubordinated debt security issued by an underwriting bank. Similar to other debt securities, ETNs have a maturity date and are backed only by the credit of the issuer.'

Presumably the operators realised barely anyone would be interested in such a thing so they decided to make it properly backed. I dunno what rights or legalities there are about withdrawing that backing.