Looks like you do need to use TWRP for this one… AOSP recovery does not allow you to disable the signature check. So you can install TWRP as laid out in that first post I linked, or use the procedure AnotherElk has referred to to boot to TWRP without installing it.
Once you’ve got TWRP up and running, you can go into its settings and disable ZIP zignature verification, and then flash. It has touch screen support so it’s a bit easier to use, too.