Warning: super technical network engineering explanation follows. But the short answer is just that it’s a bridge by definition. 
The longer answer:
A bridge (or gateway) converts requests in one protocol to that of another. The hue bridge is a Wi-Fi/ZLL Bridge. The bridge does not, however, “establish” the network. That is it does not assign network IDs to other devices that join to it or control which devices join.
Instead, because the hue bridge is a ZLL device, there is no establishing coordinator on its network.
A ZigBee Light Link network has no ZigBee network coordinator, even a bridge device merely acts as a router {repeater}
So it’s a bridge per the definition of bridge in the ZLL specification.
The end devices don’t need the bridge in order to function as a network. And they don’t need the bridge’s permission to join. This is why you can use a Hue dimmer switch with Hue bulbs without ever having to have a bridge at all.
A hub, in contrast, “establishes” a network in network engineering terms. It gives permission for each new node to join and in many protocols assigns a network ID. It owns the security keys.
No hub, no network for networks which are established by a hub.
If you have a Z wave smart bulb, and a Z wave button Device, The button device can’t talk to the Z wave smart bulb unless you also have a hub to create a network that both the bulb and the button device will join. Once both have joined the network you may be able to set up direct communication between them (Z wave direct Association) but you had to have the hub to set up the network in the first place and hand out the network addresses to each device.
I know, boring. 
Which is why no one cares about this but me and some of the other network engineering people who are community members.
But the reason the Hue device says “bridge” on the box is because by definition it is a bridge. It’s not a hub.
how.do you feel about Google calling their screen device the Google Home Hub?