It uses JSZip to open the zip file, goes through and if the filename matches '__MACOSX' it removes it. That catches the hidden folders and files.

If you want go to JSZip's demo 'Reading a file' https://stuk.github.io/jszip/documentation/examples/read-loc... and drag your original zip in there to have a look at the hidden files and folders, and then the generated zip to see what was taken out

EDIT: updated link