If that were possible then ProtonMail would have certainly said so, since that'd garner serious attention. In any case, the functionality as a whole isn't documented, so even if it works, Apple could theoretically take it away just like that. That of course doesn't mean we shouldn't rely on it, since if the only other option is a corpnet we're left with little other choice. I have the same problem with Apple because I depend on static binaries. Under the Apple regime, they too have an uncertain future and only continue to exist at Apple's pleasure. It's discomforting but not the end of the world.