That's right in essence mahamudra there is no sem/rigpa distinction. However, in Gampopa's essence mahamudra instructions, Gampopa describes everything as rigpa, "isolating the pure part," "handing" discursive thought over to "pure rigpa." Pure rigpa is arrived at through guru yoga blessings. Then, you bring discursive thought onto the path, by "looking right into the discursiveness" seeing no color or shape, it is mind's nature itself, dharmata.
I have not received Gampopas Essenence Mahamudra instructions but it surprise me that he would use the word rigpa meaning the natural state. Anyway, what you describe here above is typical Mahamudra. Shakya Sri writes like this in "The Vital Essence":
"The teachings in the Mind and Space section as well as in the Mahamudra system, tell us that the very core practice is to allow coarse and subtle thoughts to naturally vanish by looking in to their essence and letting them go in naturalness. Though this may be so it conflict directly with the view of the Great Perfection. Therefore it is extreamly important that while having recognized the natural state you allow all coarse and subtle thoughts - whatever arises as the expression of this empty and cognizant awareness - to be groundless and rootless and to be liberated simultaneous with arising, just like a wave naturally subsiding back into the water. This is the sublime and vital point." [From: Quintessential Dzogchen]
This is a very subtle point and one could perhaps find it difficult to grasp the difference, but this quote point directly to the difference between sem and rigpa that is so essential in the Nyingthik teachings.