The submitqc script we use creates the zsync file that is uploaded, but what has been seen is likely due to the missing entry of zsync.tcz in the dep file of the submitqc script, which would result in a quiet failure to generate a zsync file when used. Since my normal machines have always had zsync installed, I didn't take note of the missing dependency. And therefore anyone else installing submitqc from the repo would have been missing the zsync command if it was not already there. I added checks for certain commands at the beginning of the script, but zsync flew under the radar.
I had recently put zsync in the dep file of the submitqc script, but I did not know that so many files were out of sync. I will check the zsync file status periodically from now.