According to this link the Pi Zero is the same architecture as Pi 1:
https://wiki.debian.org/RaspberryPi
So follow the instructions on https://help.resilio.com/hc/en-us/articles/206178924-Installing-Sync-package-on-Linux and use the "For Raspberry Pi 1" section, not the "For arm64 architecture" section. In fact, I don't think the "For arm64 architecture" has to be used for any Raspberry Pi at all - as noted by brokenby2703, it installs the armhf by default anyway.
The line "sudo apt-get install resilio-sync:armel" installs the package so no need to proceed further after that.
P.S. Someone needs to clean up those instructions.