Rest of the frame looks super clean. As someone completely unqualified, you're probably fine. It's not like you ordered from tantan. Ride it, worst case, it cracks around the seat post, and you then get it replaced, just monitor the area.
I'd email the seller to mention that. saying that you expect them to honour the warranty if that questionable area ends up breaking.
i dont think it makes sense to try and get the frame replaced, it seems unreasonable.