Error: "extension/package.json not found inside zip." when repackaging and installing VSIX

I've downloaded a VSIX extension for VSCode here:

https://marketplace.visualstudio.com/items?itemName=mblode.twig-language-2

I've then tested this VSIX in VSCode and it installs, no problem.

At this point I then uninstall, rename the VSIX to a zip and then unzip to make edits.

The only edit I'm making is adding Indent rules in the Language config.

I then rezip (using macOS Finder) and rename to a VSIX file extension.

Upon trying to install again in VSCode I get the error "extension/package.json not found inside zip."

There is one at that path so I'm uncertain what the issue is.

Help appreciated.