The funny thing is that in most Windows tools, it doesn't mean even alternate file streams. It simply is flat out illegal (unless used as a drive letter); most tools will reject any program trying to save a file in the format of "filename:ADS".

Even in managed code tools, it's this way. For instance, in C#, a statement like this:

File.WriteAllText(@"c:\test.txt:teststream", "AFS test");

Will error with "path format is not supported". There's no way to access ADS natively in .NET. The only way to access ADS is to evoke native Win32 methods.

PowerShell can access ADS easily, so presumably there's something in .NET for that.

e.g. Get-Content -Path foo -Stream bar

I can't look at PowerShell's assemblies, but they could just call into native code for that.

These are things that seem normal to me that a Windows user might also describe as "weird", just like the GP post described colons in filenames.