that is true. its because brawl has a buffer to make things easier. but trust me, with some practice, it IS doable.
essentially the concept is that you are cancelling the very beginning of your dash attack with an upsmash. you will first need to change the control scheme so that you can realistically perform the sequence though. i personally change z to attack instead of grab (some chars can do it with z set to grab, most cannot though and need it to be attack), but some other people prefer changing R to attack or L to attack, just some button that can be set to attack other than the A button (it will not work with the A button since your fingers are not positioned well enough to perform the technique in that manor). what you then do is this:
tap to dash > dash attack (c stick down) > flick up on control stick and press attack button (z in my case)
if you use a quarter circle motion on the control stick, its the most efficient way to go from doing a dash attack, to pressing up and z immediately after you input the command for dash attack.
so overall, its a three step process: the quarter circle flick, the input for the dash attack (performed at beginning of flick), and the input for the attack (performed at the end of the flick, when control stick is facing upwards). you should get a large slide out of this, there are youtube videos explaining it as well if you need additional help.