Unless I am missing something and Siren and Nergal is the same person, then no, it is not a sequel to A Zombie's Life, it may well be a sequel to a Zombie's Retreat, sure, I still don't get the anime protag but whatevs. I don't really get what is hard to understand about this, again, unless they are the same person.
Oh my apologies, I thought you were one of those people complaining about the second game having the same premise as the first. I assume you're also very upset at all the incest games on this site? I mean they're all the modern world where you fuck your family? Because they all have the same premise? Or are you mad that most world war 2 games have the same setting?
Nergal basically abandoned Zombie's Life at one point (started in 2016). Siren started Zombie's Retreat, because AZL was
abandoned, with the same themes (2018). This project also has the same artist as AZL, mostly because everyone but Nergal wanted to continue the project. Years later Nergal
came back and, reluctantly, did the final parts of AZL (2019). Nergal has no desire to make another Zombie's Life game or continue and only made the final parts because people complained. (I still haven't been able to finish AZL due to bugs and crashes)
Now, in the year 2021, when there's already a full game of Zombie's Retreat out, you come out and complain that the second Zombie's Retreat has the audacity to have same premise as the first Zombie's retreat? The absolute gall of the developer, to just make a game that continues their very succesful game. If this was your main problem, you really should have spoken up when Zombie's Retreat one came out.
If you only take your games 'pure', you're definitely allowed to wait until Nergal makes another game in this setting and completely ignore this one. But buddy, I think you'll be waiting a long, long time.