That is correct, the ACS758-xxx is, in effect a shunt, with the current sensing element as a hall effect sensor.
I have used these and am quite impressed.
I am currently (pun, yes, I know) doing an update to my renewable energy system, with new Lithiums on the way and I am wanting to monitor many things, such as solar input, micro-hydro input and inverter output.
I looked and thought hard over the ways in which I could measure the current, and while using the old traditional method of 'copper' shunt with op-amp had some appeal to me, I kept coming back to thinking that using a device like the ACS758-xxx was going to be far easier to install and connect to my control system.
With these ACS devices, you can get both uni-directional as well as bi-directional.
Uni-directional devices output a voltage signal from 0v to 5v, with 0Amps giving 0 volts, and full scale giving near to 5 volts.
Bi-directional devices can measure the current flow in both directions and so they sit at Vcc/2 or 2.5 volts for 0Amps and give lower values for lower amps and higher volts for higher amps.
For me, I prefer to use uni-directional so I can get the full scale range over 0v to 5v for reading into the analog input.
Some of the smaller devices, such as the popular ACS712-xxx are bi-directional.
Hope what I wrote makes sense for you.