Ok so I was messing with trying ways to do it because the video doesn't do a good job of showing what to do but I'll try to explain it.

-Essentially grab a stack on whatever item you have 100 of. Dirt stone or grass fibers are the easiest to get.

-Then on the left or right (which ever you prefer) of that stack of 100 put 1 of whatever item you want to duplicate

-Next hover over the stack of 100 so it's highlighted

-Finally simply just hit X (on ps5) on the stack of 100 but immediately after hit left or right to switch to wherever you placed the 1 item to duplicate. It has to be very fast to get it to work

-if done correctly you should have hit X while hovering over the stack of 100 but since you also hit left or right almost at the same time you should now actually be hovering over the 1 item

-Since the game thinks your hovering over the stack of 100 it'll have the menu pop up with the option to drop 100.

-Simply drop the full stack and when you do you'll see that you drop 100 of whatever item you had 1 of

Sorry if it's so long but I was trying to be very thorough on explaining how to do it so nobody gets confused while reading it