I thought it used an ethernet cable to plug from the home hub to a special ethernet connecter on the master socket?
The NTE5 is largely a dumb mechanical object, it doesn't turn anything into Ethernet, and the signal coming into the customer's property most certainly isn't Ethernet when it gets there. The most complicated piece of electronics is a simple low pass filter to remove the VDSL frequencies from the signal going to the analogue phones in your home. This is much like the filters every DIY store has carried for the past decade or so, except better made and guaranteed for the VDSL frequency range.
What confuses people is that the socket used is narrower than the "Type 600" socket provided for plugging in a conventional analogue telephone, largely to provide a visual clue as to which socket is for what and so it looks more like the sockets they're familiar with in home or office networks. But that's not what it is, don't try to plug a pre-made network cable in because it won't fit.
You could argue for using Category 5e unshielded twisted pair cable (what you might call "Ethernet cable") to connect the Home Hub to that socket anyway, just wiring the appropriate connectors onto each end - much as people have been known to replace all their internal telephone extension cables with Cat 5e cable. But the resulting improvement in signal quality would probably be too small to detect unless you for some reason are running that short cable past the back of a fridge or some similarly EMF-hostile environment. If you want to, go ahead, Cat 5e is very cheap (because widely used) and it can't hurt to do this if you get the wiring right. But it's no surprise that BT decided not to spring for the fancier cable.
Early ADSL subscribers got these split NTE5 designs too, before ADSL was self-install and micro-filters were as common as bottle caps, I remember being very excited by what seemed to be an Ethernet socket, and then rather disappointed when I realised it was only done that way to reduce the chance of me plugging it in wrong.