Well I agree that it could still be our own galaxy. It's not meant to be an exact representation of all spiral arms and it's irregular extensions but it rather only shows the 2 main arms which originate from the galactic centre, which is the Perseus Arm and the Scutum-Centaurus Arm.
https://upload.wikimedia.org/wikipedia/commons/a/a7/Milky_Way_Arms_ssc2008-10.svg
Agree. It also meshes nicely with the Formadine Rift mystery... The spaces between the spiral arms are literally not on the Thargoids' map.