Exactly. It's pretty rare that you are checking a collision at your exact position, so the argument passed in is usually your position + an offset. But more importantly is that we are combining this with bbox_ variables. Since bbox is also a variable that is always "relative" to the x position of the calling object, combining it with the offset gives us final range of tiles that needs to be checked for collision.
Viewing post in Converting TDMC to use Tiles comments
Cheers! I think what confused me was the conversion of the script into the 2.3+ format. I replaced argument0 and argument1 in the function declaration with x and y (as that's what you had in the @param comments) and that changed the x and y already in the script to be script variables instead of instance variables (and thus, would always result in zero for the aforementioned calculations). All good now!