The problem is
With Luffy's you can just
Hold Down
And hold the Special Button
and he still goes up, twice

With Sanddash, you have to go Down-Special Button-Neutral-Up real darn fast in order to get any height at all
And using it at the flick of the wrist is a lot harder than it seems given Sanddash's timing (If you're too slow, you don't move anywhere at all and just wasted your main method of recovery) and trying to do it on a Keyboard is hell

If you're using a Keyboard you can just hold up and down at the same time and it'll go up lmoa.

Why exactly does this trick not work for me?

Since when? Even just doing training with bowser set to "evade", bowser can easily DI out of it at 35%, I'm not able to hit it unless the bowser doesn't DI and doesn't tech. Up to 34% though, you are right, Bowser seems probably just heavy enough and big enough to get hit by it. Although, at this point, dthrow->uair->bair->bair is stronger and dthrow->dair does similar damage, both not doing any damage to yourself

Keyboard is best way to play Sandbag tho.
Also I bombed myself a good majority of those times you said to do it at the same time, lmoa.

some guy tried to troll me by picking sandbag, set the Star as the only item while setting item spawn to none. He spammed neutral B to give himself invincibility with the star and kept spamming Up B so that he could kill me early while still not taking self-damage. He was clever, but in the end I still won

I like the way that dude thinks.

oh s*** now i know how to annoy my friends thanks dude

Just replace this with the Home-Run bat for the greatest Rest punish of all time.

Question: can dthrow->nair legitimately chaingrab even if being somewhat DI dependent.

No.

My friend (who is a main) thinks the only reason I'm better than him is because I use sandbag, then I used and wiped the floor with another friend ( main was in the same discord call other friend is a main) he realized that I was better than him.

Don't know what that has to do with anything, heck, It should be in the board.

I don't think that belongs here.

i just knew how to use sandbag's down special