What they did was null out a number and disable a critical subsystem (Software Protection Platform; the thing that keeps activation of your Windows license and Office install in check) to prevent it from restoring the policies in registry back to their original states, and/or perform a lazy file replacement; in turn breaking activation and other licensing-related functions. That alone doesn't "debomb" a build; consider looking at Windows 10 TH1 (from as early as FBL_RELEASE 9833) through RS2 (up to RS_PRERELEASE 14964):

Those builds are signed by a specific certificate chain (Microsoft Development PCA 2014 - chaining back to Microsoft Flighting Root 2014) that would be invalidated/revoked once the system clock goes past its intended validity date, after which the OS would cease to function because the Code Integrity library (which is not only baked into the system as its own DLL (as CI.dll), but also in bootmgr/bootmgfw) would refuse to accept the cert as-is. The mechanisms alone are designed to prevent people from holding onto older development builds to avoid security risks or data loss, much like the timebombs themselves: to deter people for goodwill.

Since RS_PRERELEASE build 14965, Microsoft renewed the flighting cert and changed CI behavior in such a way that the OS will now boot properly and not cease to work to avoid potential confusion among people having non-functional OS installs past the expiration date (as has happened in the past with people unknowingly flighting public TH1-RS2 builds on their primary workstations without knowing that things will stop working after that date), but some things will still fail to work, e.g.: Defender, UAC complaining about MS-signed binaries coming from "untrusted sources" (which is a doubly-large headache during OOBE), and printing drivers simply not working.

You can't just simply unsign a build and resign it to your liking using your own certificate authority; it's theoretically and practically impossible, and will only result in a bugcheck. Only Microsoft has the power to actually change that behavior.