I'd say it's because Proton does not employ actual cryptographers or security engineers. They do seem to have a lot of marketing folk, interns, and particle physicists, allegedly.
I love this but just looking at the comments - while admittedly cool, implementing your own OS isn't black magic or particularly arcane knowledge; sure it's a whole lot of work where you're often reimplementing the wheel, but not outside the realm of any reasonably skilled graduate I would think.
> Marketplace developers must provide Apple a stand-by letter of credit from an A-rated (or equivalent by S&P, Fitch, or Moody’s) financial Institution of €1,000,000 prior to receiving the entitlement. It will need to be auto-renewed on a yearly basis.