No, he worked on Signal for so long that he probably just wanted to take a break to work on other passions too - he's still on the Signal Foundation board (https://signalfoundation.org/)
> Why is a secure messenger adopting the appearance of social media?
I'd argue that Stories (or equivalent) is nowadays a standard feature in many messengers. To more directly answer your question no - Signal is missing a key aspect of Social Media: discovery. Stories is pretty much equivalent to share a picture to a group of people. You can also easily disable the feature in settings.
> Doesn't this work explicitly against the entire claimed reason for not having an account system?
I'm not sure specifically to what you refer to but in general: phone numbers is still the primary way to find new folks, but they're working on a username feature. They will still use phone numbers for simplicity as "account" but again, Stories is simply a new interface to share pictures with your contacts.