If Sunder replaced any melee attack, there would not be a standard action entry for Sunder on the table, because you would never take a Sunder action. If you had a standard action available and wished to Sunder, you would take the Attack action. If you wanted to move in a straight line and Sunder, you would take the Charge action. If you wanted to Sunder multiple times in a round, or combine a Sunder with other attacks, you would take the Full Attack action. You would never take a Sunder action, so the action would not exist and would not appear on the table. Further, in this case, the usage of Sunder would be identical to Disarm, Trip, and Grapple; it would appear with those special attacks as Action Type: Varies, and bear footnote 7.
Since the Sunder standard action (which would never be used if Sunder replaced any melee attack) exists, and since it does not bear footnote 7 (as it would if Sunder replaced any melee attack), the only conclusion I can draw is that Sunder does not replace any melee attack.
The text still applies; however, the phrase "You may use a melee attack to..." does not mean "In place of any melee attack from any source, you may..."
The existence of the Sunder standard action on the table tells me that that reading of the phrase is incorrect. I'm not ignoring the text; I'm just taking note that it doesn't mean what you think it does.
-Hyp.