He deleted the signing keys because it looked like the other owner of Copperhead OS wanted to make the signing keys available to government agencies and/or criminal organizations. He deleted the signing keys to protect their users against malicious updates, which is the right thing to do and should increase trust in him and the project.

It's worth actually reading the linked post. Relevant segment:

In 2018, matters between Micay and Donaldson came to a head over Donaldson’s desire to pursue business deals with criminal organizations, and his attempts to compromise the security of CopperheadOS, including by proposing license enforcement and remote updating systems that would allow third-parties to have access to users’ phones. As part of this process, Donaldson began to demand that Micay provide Donaldson with the “signing keys” - i.e. the credentials required to verify the authenticity of releases of CopperheadOS. Donaldson advised that, in order to secure certain new business, potential customers required access to the Keys.

The keys had been in continuous use by Micay, in his personal capacity, since before the incorporation of Copperhead. However, more importantly, any party with the keys could mark malicious software as “authentic”, and thereby infiltrate devices using CopperheadOS.

Micay was unwilling to participate in that kind of security breach. Since Donaldson had control over certain infrastructure for the open source project, he would be able to incorporate (or hire others to incorporate) the privacy-damaging features described above for all future releases of CopperheadOS. Micay therefore deleted the keys permanently and severed ties with Copperhead and Donaldson.

Ah, thanks for setting me straight. That's reassuring. I think I would still have more respect and trust for GrapheneOS if they either didn't respond, or struck a more neutral tone; but that's more subjective.