Can anybody demonstrate a for loop with a slice? like mentioned at the end of the video

He simply meant that slices can be used as a source for a loop just like a string or list. So if I wanted to print the 6 last characters of a string for example I could use a loop like this:

example = "Hello World!"

for letter in example[-6:]:
  print(letter)

Now that is not a very useful loop but it would work, and that is what Kenneth meant, if slices were not considered iterable then they could not have been used as a source for a loop at all.

And his negative step example looks something like this:

example = "Hello World!"

for letter in example[::-1]:
  print(letter)

That would print the string in reverse since it would be stepped through starting from the end and moving backwards. Both of these examples would of course also work if a list were sliced instead of a string.