looks cheap and …
looks cheap and time consuming, it would be faster and easier to put the countertop on the cabinet and scribe it

To time consuming, …
To time consuming, be an old man by the time you finish. XD

It has compression …
It has compression rivets that hold nicely. You have to bump it hard to mess up the template. I am willing to bet you have not used one

this thing could …
this thing could easily move on you and give you an inaccurate mark.

This was made as a …
This was made as a concept video. It does not show everything exactly as it could because it was made with what was at hand at the time.

it’s a nice idea. …
it’s a nice idea. but a bit gangly looking and then the long wall still appeared to be 3/16-1/4″ gap.

That will give a …
That will give a parallel reference to the wall but not a square reference to the whole cabinet. This is a multiple step process to get it close.

AngleBuster makes it a one simpler.

Old ways die hard.

how about placing …
how about placing the counter top against the wall the take a compass and scribe the line.