If another capable dev was brought in to work on it independently, I think I would help to transfer it to them, and quit afterwards. When the dev would not be capable, I would advise against it. But in this case, it is the decision of the company owners, and they turned out to not be susceptible to any arguments against their decision. They also see themselves as "developers" right now, with the help of the tools they use. Focussing on limits and boundaries in writing is a good point, I will review this where needed if I continue.