I'm back after my first answer failed miserably.

Answer Number 2

You have no idea how excited I am about this. Years after learning about the Xor operator and I finally get to use it!!

VBA - Logical Operators: Xor Operator

Called Logical Exclusion. It is the combination of NOT and OR Operator. If one, and only one, of the expressions evaluates to be True, the result is True.

The Xor seems like a great fit because both the parameters need to be tested for nulls and the values can not equal one another.

My Version

Public Function NotEquals(LeftValue As Variant, RightValue As Variant) As Boolean
    If IsNull(LeftValue) Xor IsNull(RightValue) Then
        NotEquals = True
    ElseIf LeftValue <> RightValue Then
        NotEquals = True
    End If
End Function

Results

Crude Tests

Option Explicit
Public Const Repetitions = 10000000

Public Function IsDistinct(LeftValue As Variant, RightValue As Variant) As Boolean
    If IsNull(LeftValue) Then
        If IsNull(RightValue) Then
            IsDistinct = False
        Else
            IsDistinct = True
        End If
    Else
        If IsNull(RightValue) Then
            IsDistinct = True
        Else
            IsDistinct = Not (LeftValue = RightValue)
        End If
    End If
End Function

Public Function NotEquals(LeftValue As Variant, RightValue As Variant) As Boolean
    If IsNull(LeftValue) Xor IsNull(RightValue) Then
        NotEquals = True
    ElseIf LeftValue <> RightValue Then
        NotEquals = True
    End If
End Function

Sub CompareResults()
    Dim SumOfTime As Double
    Debug.Print "Number of Repetitions: "; FormatNumber(Repetitions, 0)
    
    Debug.Print
    Debug.Print "IsDistinct Test"
    Debug.Print "Left"; Tab(8); "Right"; Tab(16); "Time"; Tab(24); "Result"
    SumOfTime = TestIsDistinct(1, 1)
    SumOfTime = SumOfTime + TestIsDistinct(1, 0)
    SumOfTime = SumOfTime + TestIsDistinct(Null, 1)
    SumOfTime = SumOfTime + TestIsDistinct(0, Null)
    SumOfTime = SumOfTime + TestIsDistinct(Null, Null)
    Debug.Print "Total Time for All IsDistinct Test: "; SumOfTime
    
    Debug.Print
    Debug.Print "NotEquals Test"
    Debug.Print "Left"; Tab(8); "Right"; Tab(16); "Time"; Tab(24); "Result"
    SumOfTime = TestNotEquals(1, 1)
    SumOfTime = SumOfTime + TestNotEquals(1, 0)
    SumOfTime = SumOfTime + TestNotEquals(Null, 1)
    SumOfTime = SumOfTime + TestNotEquals(0, Null)
    SumOfTime = SumOfTime + TestNotEquals(Null, Null)
    Debug.Print "Total Time for All NotEquals Test: "; SumOfTime
    
End Sub

Function TestIsDistinct(LeftValue As Variant, RightValue As Variant) As Double
    Dim t As Double
    t = Timer
    Dim Result As Boolean
    Dim n As Long
    For n = 1 To Repetitions
        Result = IsDistinct(LeftValue, RightValue)
    Next

    Dim SumOfTime As Double
    SumOfTime = Round(Timer - t, 4)
    
    Debug.Print LeftValue; Tab(8); RightValue; Tab(16); SumOfTime; Tab(24); Result
    
    TestIsDistinct = SumOfTime
End Function

Function TestNotEquals(LeftValue As Variant, RightValue As Variant) As Double
    Dim t As Double
    t = Timer
    Dim Result As Boolean
    Dim n As Long
    For n = 1 To Repetitions
        Result = NotEquals(LeftValue, RightValue)
    Next
    
    Dim SumOfTime As Double
    SumOfTime = Round(Timer - t, 4)
    
    Debug.Print LeftValue; Tab(8); RightValue; Tab(16); SumOfTime; Tab(24); Result
    
    TestNotEquals = SumOfTime
End Function