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.
There is a large segment of people who don't like complicated passwords and will not use a password manager, so the PIN scheme w/ advanced options and password manager integration is the way to thread that needle.