Weaponization is the stage of the Cyber Kill Chain where the attacker creates or modifies a malicious payload to use against a target. In this case, the disgruntled open-source developer has created a logic bomb that will act as a wiper, which is a type of malware that destroys data on a system. This is an example of weaponization, as the developer has prepared a cyberweapon to sabotage the code repository.
References: The answer was based on the web search results from Bing, especially the following sources:
Cyber Kill Chain | Lockheed Martin, which states: "In the weaponization step, the adversary creates remote access malware weapon, such as a virus or worm, tailored to one or more vulnerabilities." The Cyber Kill Chain: The Seven Steps of a Cyberattack - EC-Council, which states: "In the weaponization stage, all of the attacker's preparatory work culminates in the creation of malware to be used against an identified target." What is the Cyber Kill Chain? Introduction Guide - CrowdStrike, which states: "Weaponization: The attacker creates a malicious payload that will be delivered to the target."