These extension methods exist for CQLinq friendly syntax reasons and can only be called in a ICQLinqExecutionContext.
Indeed they all rely on a hidden NDepend.CodeModel.ICompareContext object, hidden in CQLinq queries. This results in less syntax burden.
At CQLinq compilation time, the CQLinq compiler takes care of transforming calls to these extension methods into calls to methods of a NDepend.CodeModel.ICompareContext object.
Indeed each of these extension methods, has an equivalent method defined by NDepend.CodeModel.ICompareContext, that can be called from a program that consumes NDepend.API.

Reference

ExtensionMethodsCQLinqCompare Members
NDepend.Reserved.CQLinq Namespace
ICompareContext Interface
CreateCompareContextWithOlder Method
RelyOnCompareContext Property
Compile(String,ICompareContext) Method
CompileMany(IEnumerable<String>,ICompareContext) Method
TryCompareSourceWith(ISourceDecl,ISourceDecl) Method