Outside of that pond, there is a giant ocean of developers that work in enterprise and make a perfectly respectable income.
You can learn that on blazor and apply it elsewhere.
That being said I have worked with blazor since launch in 2019 and it's one of my least favorite SPA frameworks.
I went into [some detail](https://news.ycombinator.com/item?id=37798121) on another comment on this thread.
Having said that, first job is a first job. Take that c# you learn to a REST backend job once you have a couple years experience.
I agree with them on Blazor and I agree with you on dotnet (e.g. WebAPI) too. The two positions aren't mutually exclusive.
Being a Blazor developer will definitely be career limiting. I'd take almost any other skill set on a new hire (e.g. Angular, React, .Net MVC, Java, et al). The only thing it may beat is Web Forms/Visual Basic, because at least you'll know modern C# syntax. But I'm not allowing Blazor in any codebase I control.