So yeah, it turns out my device was always rooted. As Robert said, I probably broke something that SuperSU used.
If anyone finds this helpful, I got it back to normal through recovery, like Izzy said.
I had previously backed up the build.prop file to my SDcard, so I created a zip file with
build.prop and a
meta-inf folder that I found in a custom ROM (I actually found it in this question from the XDA forum https://forum.xda-developers.com/t/copy-build-prop-with-flashable-zip.2986827/
However, this https://www.reddit.com/r/FirmwareDevelopment/comments/1iut7m/howto_understanding_the_metainf_folder_for/ says you can get it from any custom ROM). Flashed the zip file through CWM recovery and after rebooting, my phone was acting like nothing ever happened c:
*If you didn't backup
build.prop, I'm sure there's ways to retrieve it through ADB, pretty sure you can copy it and paste it somewhere. Either way, you can still see the content of the file in Android Studio, and I'm guessing Eclipse and other programs I don't know of; and I'm a n00b, but I'm pretty sure you can make a brand new build.prop by copying the contents of the existing one on a text editor, of course changing what you had previously modified, hence leaving it the way it was before you messed it up.