Taking a current gen game and dialing it back to last gen doesn't mean it's running current gen.
The problem is that the terms "current gen" and "last gen" are fairly arbitrary. In addition, using the scaling down of certain games to have it run and the inability of the hardware to run certain games does not mean it is "last gen". By the token of your argument, a game like Doom is not current-gen on a system like the base PS4 because it can run at much higher resolutions and with better graphical effects on a mid-to-high end gaming PC. Beyond this, what about the many games that are not really graphically intensive but released only on recent consoles? Are they last-gen as well?
Also, how could you be sure that Doom and Wolfenstein II would run on a last-gen system. The Switch's raw power in handheld mode might be comparable to that of a last-gen stationary system, but its GPU architecture is fairly modern and matches the features on the current-gen stationary systems. Engines that rely on software APIs base around the newer hardware features might not scale to PS3 and Xbox 360 but might do so on Switch because the GPU has the necessary features for the engine to run.