I tried to reproduce the error that you are seeing on a
Windows 10 VM with vscode 1.60.0 and I never get the missing command error.
Also, after learning a bit about vscode extensions and how they work, the only way for you to have the issue is if the file wasn't properly recognized as using the amxxpawn language. As soon as the file is recognized as or set to use amxxpawn then the extension will get activated and register those commands. Note: The activation events is for activating the extension (including commands), not the individual commands themselves.
In summary, I suspect that your file might not be properly set to the amxxpawn language.