The SHA-2 checksums to verify packages against are delivered as part of the (signed) package index (as the article alludes to).
usign: https://git.openwrt.org/project/usign.git
ucert: https://git.openwrt.org/project/ucert.git
Firmware releases are also signed with GPG: https://openwrt.org/docs/guide-user/security/release_signatu...
openwrt/openwrt: https://github.com/openwrt/openwrt
openwrt/packages: https://github.com/openwrt/packages
openwrt/openwrt/search?q="usign" https://github.com/openwrt/openwrt/search?q=usign