Wrong file. The instructions were to add the line to the .vmx file, not the vmxf file.  Remove the line from your .vmxf file if it’s still there  

Since you are using Fusion you’ll get a better answer in the VMware Fusion Discussions forum for questions about Fusion as this forum focuses on VMware Workstation on Windows and Linux  

It’s strange why you could not find the .vmx file. It is normally found in the VM’s bundle directory, the same place where your virtual disk files and the .vmxf file are found. 

But you should not have to directly edit the .vmx file to disable side channel mitigations in Fusion 12.x (the only supported version for macOS Monterey 12.x). The setting is in the GUI. Open the VM’s Settings, click on the Advanced settings and you will find a check box to disable side channel mitigations.