That link to the altium website is about reverse generating a PCB from CAM output. Not about creating a schematic.
KiCad has a feature in which netlist data can be back ported to the schematic, and it can create wire labels attached to schematic symbols. I have not used this feature myself, and don't know how good it works.
KiCad can import an altium PCB (or directly import Gerbers). And apparently altium can (or could?) import a KiCad project. I think that may be limited to the KiCad V6 file formart. I don't know if altium put any effort into keeping that feature of their softwar up to date. KiCad has just released V9.
A method that always work, is to manually enter a schematic, and link them to the already existing PCB footprints. You can then "guess" the schematic, and differences will be flagged by DRC once the schematic is ported back to the PCB. Often quite big parts of a schematic can be easily guessed. This is an iterative process and thus takes some time