If you believe the device is soft bricked, you are confirming it if you really cannot connect to fastboot, then I believe you will need to get access to Windows.
Here are two links in case this helps give a different perspective from the youtube link.
- https://community.oneplus.com/thread/1541327
- https://xdaforums.com/t/op8-oos-21aa-ba-da-unbrick-tool-to-restore-your-device-to-oxygenos.4085877/
Re Windows, in another forum post the possibility of a Windows VM on cloud was mentioned https://www.cloudbik.com/resources/blog/create-windows-virtual-machine-azure/. Reference here.