> You seem knowledgeable about this, so I'll take the opportunity to ask: if I install a malicious app and it manages to escape the sandbox and alter the system, my understanding is that it will be detected next time I boot it (because the image hash won't match). Isn't that true?

They're misrepresenting what has been said by GrapheneOS and also lack a good understanding of it themselves. They're definitely not a good source of information about this.