Only allow TLS 1.2 on kestrel

I want to disable all protocols that are older than TLS 1.2. On other posts I read that I can configure it like this:

WebHost.CreateDefaultBuilder(args)
    .UseKestrel(c =>
    {
        c.ConfigureHttpsDefaults(configureOptions => 
        { 
            configureOptions.SslProtocols = SslProtocols.Tls12; 
        });
    })
    .UseStartup<Startup>();

When I test this setup with ssllabs I see that older protocols are still supported. It seems that I can only configure the defaults, but I find conflicting blogs/posts that claim that I should be able to only allow TLS 1.2 this way (like this blog).

Am I missing something or is it really only possible to set the defaults on Kestrel?

Note: I am restricted to AspNET Core 2.2 because I can not (yet) move from .NET Framework to .Net Core.