Thanks again @flug32! Because of this sub‑sentence of yours I have just realized and understood what the TimeWarp video mode does. 
Yey! It simply records video at different fps speeds below the nominal fps speed, like 25 fps (for PAL) or 29.97 fps (for NTSC) etc. and stores it in a video container set to the nominal fps speed. In other words, it drops frames from the standard video feed but sets the playback speed to nominal fps. Sheesh 
that “TimeWarp” marketing term and the use of N× speedups in the UI are awfully misleading and difficult to understand.
Well, I guess this is GoPro style; why make things easy if you can make them overly complicated. They should have rather simply let the user choose the recording fps and the playback fps of the video container (or stream) separately. Just call it “Recording FPS” and “Playback FPS” in the UI. That’s it. Done! 
But no, let’s invent a convoluted marketing term! 