Either way, the error ended up being me not understanding how to use nodemcu-flasher. I kept copying files into the same folder with it and renaming them to nodemcu_512k.bin like some sites were suggesting (apparently that is how it used to work), but it just kept reflashing the 20150105 version (with the socket send bug), because that is the version stored internally in the flasher. Instead, you should follow the instructions here: https://github.com/nodemcu/nodemcu-flasher under setting your own firmware. Click the top gear icon and choose the nodemcu_latest.bin you downloaded from here: https://github.com/nodemcu/nodemcu-firm ... n?raw=true and blank out and clear the x on the other entries. Now you are really flashing the latest version.
Maybe everyone else in the world has already figured this out, but if not, maybe it will help someone.