Yes that's BT Wholesale based backhaul, rather than Zen's own GEA backhaul.
You may want to check this with Zen; that there aren't any mistakes at their end with authentication, if you had been provisioned (or intended to be provisioned) onto Zen backhaul - especially with authentication issues like you appear to be having.
What would be the correct question to ask Zen? Something along the lines that “the BT Wholesale Checker indicates that I’m on BT Wholesale Backhaul. Is this correct? Or should I be on Zen backhaul? “
Ah no, no no - I think you misunderstood, and I think I misunderstood you.
This information on which Zen backhaul you have is not available from he BTW Checker - this is internal to Zen and any notes etc about it would be on your Zen account portal - or from speaking directly with Zen.
Zen will choose how they will get your traffic from the exchange back to them (this is the backhaul) - they will either use BT Wholesale to provide this connection or indeed their own GEA backhaul.
This is not to be confused with the information on the publicly available BT Wholesale checker tool.