that's not what he means decoding 4:4:4 needs double the memory as 4:2:0 the image is now 2 times the size it is as it is.
and full range has other issues as long as YCbCr is used it can create out of range values this could be "fixed" with YCoCg or ICtCp. RGB is not an option.
the real reason no one fixed this is 4:2:0 is good "enough" for these casual professionals that create charts like these:
https://www.unravel.com.au/understanding-gamma