Earlier quoted context omitted.
Isn't the subtext of this post pretty clearly that the unauthorized actor was Andre Arko, who had until days prior all the same access to RubyGems.org already? The impression I have reading this is that they're going out of their way to make it clear they believe it was him, but aren't naming him because doing so would be accusing him of a criminal act.
Let's say that they are 100% correct, we parse the subtext as text, it was totally him. We still do not know the critical details of how (and when) he stored the root password he copied out of their password manager (encrypted in his own password manager? on his pwned laptop? in dropbox? we'll never know!) therefore the whole chain of custody is still broken.
Why are you copying a password out of a shared vault that should only be used in break-glass type scenarios? It's that's not planning for possible malicious action in the future, I don't know what is.
You can try and excuse it as having your own break-glass for the break-glass, but that's on the spectrum between irresponsible and incompetent.
Again, if the accusation is true, removing him was justifiable from any possible perspective you might have.