This makes perfect sense. I'm using a kitchen scale to measure the 2-part silicone mixture that I'm using for making toys, not because weight is critical but because it needs to be right for curing. I should do similar when mixing epoxy, but I always eyeball that for some reason. Maybe has to do with cost, it's $10-20 worth of silicone I'm mixing, and usually a quarter worth of epoxy, just due to quantities involved.
This reminds me of how often I quote weight limits on cars to people and their eyes go wide at how easy it is to exceed the OEM's recommended limits. I'm fairly sure I'm one of the few among my friend groups that has read through every owners manual for the cars/vans I've owned.
I think the commenter was making the point that the weight of the paint is not a significant consideration, not that recalculating the weight and balance after a paint job should not be done.
Painting a plane is one of those times that you often strip everything out anyway, so it's a convenient time to check the weight and balance against the logbook.
I used to work in general aviation. If my eyes could fly a loop in my skull they would have.
Were the occupants required to use the bathroom before flying? That's how much weight you're potentially saving by weighing the paint on a small aircraft.
They make you weigh the paint because they want you to spray on a certain thickness so they say "X oz paint, Y oz thinner/hardener" (or something like that) in order to get your mixture into the right ballpark so it will work with whatever procedure they want you to spray it on with and get the thickness/finish/hardness the OEM wants you to get.
In aviation there's a ton of treating simple systems as black boxes and "do X and exactly X" type maintenance that happens in order to smoothly transfer liability. You paint a cowl the way the OEM says not because you couldn't get an equivalently performing cowl a different way but because you don't want the NTSB coming after you trying to determine if you did it different but right or different but wrong.
The specifications to which general aviation stuff is done isn't really any more exacting than stuff in automotive or heavy industry. The service literature is just more verbose and the service procedures are more tightly defined.