rafael3d

That's true- JPEG supports 444, 422, 420, lossless, etc. However when comparing images the compression level needs to be the same (most importantly, the DCT quantization). The rescaled web images are likely 420 while the downloads could still possibly be originals. The point about 420 is that web-facing content is typically 420 (with rare exceptions due to the desire to conserve bandwidth and save costs).
 
If at 100% A/B cycling we can't see any difference, then there's nothing gained since the results aren't visible (except when blown up or via difference analysis, etc.).
 
Scaling an image down (with e.g. bicubic) then scaling it back up acts like a low-pass filter (smoothes out macroblock edges and reduces noise). I downloaded the 4K 420 image and scaled in down then up and A/B-compared in Photoshop- couldn't see a difference other than the reduction of macroblock edges (low pass filtering) etc. Adding a sharpen filtering got both images looking very similar. A difference compare was black (not visible), however with gamma cranked up the differences from the effective low-pass filtering are clear.

The compression algorithm it's not important here. I did some synthetic tests (>post #118) simulating a best case scenario (a full 4K 4:4:4 8-bit image to a 2K 4:4:4 32-bit). I've tried different scaling algorithms and even adding noise doesn't give us any extra colour information. 
  We technically get an image with increased bit depth(since we're blending some pixels), but it doesn't translate in smoother tonality or better colours.  
 

Just to double check my months earlier results, I found a shot with some blue sky with banding that I took 2 weeks ago.  (4k 4:2:2 8 bit) I saved one copy as a 16 bit tiff.  I saved another exactly the same but downscaled by 50% using bicubic in ps.  I then opened them in ps and toggled between the images, one viewed at 200%, and the other at 400% so they would be the same enlarged on screen size.  The banding looked exactly the same in both and only the ever so slightest change in contrast which makes it appear a hair sharper., but really almost undiscernible. I then applied the same amount of saturation to each image to see if i could notice a difference in tonality.  Zero. One thing I must admit i forgot this time around is making sure the scaling was done in linear gamma.  I can't remember if photoshop has this as a default.