It is a distinction largely because you won't study to become a licensed taxi driver to only drive for Uber (or Hailo or whatever). You are right that it does not automatically make it difference, but in practice it means you're almost always dealing with drivers that are genuinely going to be taking jobs from more than one source (if nothing else by picking up clients kerb-side) because that is the main advantage of their license.

Part of the argument in the judgement is that Uber in effect acts as the client, since their rules prevent the driver from interacting with the rider in a way a independent contractor would normally be ble to. That makes it all the more significant when Uber is often the only client. For someone like Hailo this would be less risky as the driver also works for many others most of the time, so even if Hailo was found to be the client rather than the individual riders, they could make the argument that to most their drivers they were just one of many clients.

That Uber in "ideal" circumstances could have only signed up drivers that drive for many others too, or that Hailo in theory could have only signed up drivers that were only driving for them, isn't relevant when that's not how things have been in practice.

EDIT: I'd very much like to hear arguments instead of downvotes, given that my comment is entirely factual. Downvoting doesn't change any of it.