If they had a team that knows nest and can iterate fast with it that's the perfect choice. I've worked at multiple agencies over the years and was mostly in C# teams. Whenever a C# team got on greenfield/startup projects it ended up being a shit-show of self inflicted slowdowns/over-complications. I've seen 3 projects where in large part due to slow development of the MVP the project missed investment window or ran out of funds.
From my experience Node/Rails teams were much more capable of delivering shit that works. It would eventually have a bunch of problems that would be non issues by default on a different stack like ASP.NET, but the difficulty of getting to that point and realizing that just being in that situation is a win is what most engineers miss.