Those self-contained exes need a .NET runtime to be just-in-time (JIT) compiled to code your CPU can actually run. This JIT compilation happens as your app runs, so your startup performance suffers as a result. On the other hand, bflat compiles your project ahead of time into native code your CPU understands.

Also, to build that exe you have to use the .NET SDK which pulls in lots of tooling: MSBuild, NuGet, etc... It looks like bflat ditches all of that.