Earlier quoted context omitted.
The PIN is a very short key, so the only real protection is that Intel SGX (which notably gets broken every year, and so this doesn't actually work) is preventing people from getting access to the encrypted data to brute force the key; the enclave then enforces some limit on the number of attempts (and maybe some rate limit on the speed of attempts).
The client allows them to be alphanumeric, but the default is 4 numbers. The irritating behaviour of repeatedly asking for it to be entered at awkward times means people will just set it to 1337 and call it a day.
>Because Signal doesn’t have access to your keys – or your data – your PIN isn’t recoverable if you forget it, so our apps help you remember your PIN with periodic reminders. Don’t worry, these reminders get less frequent over time.
Having read that, I doubt I'll get too irritated at the reminders and eventually they'll go away. You have a point that anyone who hasn't read the update blog will have no similar hope and might use something easy. I guess I'd hope the Signal user base is more security conscious than that. For instance, I know my PIN is going to be alphanumeric and immediately stored my password database.