Piggyback - Comparison Sites?

[Zoom7000]

So, I'm guessing we all understand the concept of MVNOs (Tesco, Lebara, Voxi, ID, giffgaff etc) who piggyback off the main 4 networks (EE, Vodafone, O2 and Three)

 

I'm with EE and I'm trying to find a decent retentions deal. When I was getting them to try and match or better one of the EE deals on uswitch, they were telling me that anyone who comes from uswitch is also put on a piggyback plan, even though the contract is with EE. Vodafone sales tried telling me the very same thing.

 

So, are they telling utter porkies here and just trying to get the direct sale, or is there truth to the deals you get from comparison sites just aren't as good as buying direct?