He is not saying śamatha can be attained in a single afternoon, is he?
And the distinction is this: Rongzom makes the point that if someone is not capable understanding the meaning of the Great Perfection and must resort to a path involving effort, then following the Mañjuśrīmitra's instruction, there are two indirect ways of entering into the knowledge of Dzogchen combined with Dzogchen view: (1) through developing a stable contemplation though standard parāmitāyāna śamatha and vipaśyanā practice, where he identifies six faults and ten obscurations of concentration and three faults related to vipaśyanā, which he explains in detail.
(2) One can practice his unique system of six limb yoga of Secret Mantra, again, combined with Dzogchen view. For example, following one's vows is considered to be pratyāhara-- one sees all the consciousnesses of the sense as fauts, but one does not abandon them. This is considered to render one a container for concentration. Concentration itself is focusing on the three secrets, the deity, mantra, and wisdom. In this context then, the five mental factor associated with concentration manifest. Once the factors of concentration arise, then one practices prāṇāyama, specifically breath retention, which produces different signs during the day and during the night, etc. It is quite detailed and interesting. He concludes, "In brief, also all these different concentrations will be skillful conduct if done in connection with the meaning of Dzogchen, but will be unskillful conduct if not connected."
No one can decide for you if you are someone who needs to practice with effort or without, but since the main point is to nakedly expose rig pa from the beginning, then, we try to achieve this from the beginning. If we succeed in this, then we do not need to follow the path of effort described above. All of these concentrations described by Rongom are considered "meditation" by Longchenpa, and mind-based, and distinct from the unique natural concentration that characterizes nakedly exposing vidyā.
Claiming however that one can only practice trekcho after realizing emptiness however, is totally in the wrong direction.