I can see how it might be hard to tell the difference, but I come from a traditional film and photography background, and once you see the flaws in certain cheap filters, you can't unsee them.
I know we're talking about drone ND filters right now, but have you ever seen a cheap vs expensive vari-ND side by side?
This video is outstanding for showing all the problems with almost ALL various-ND's, and why I try not to use them (or Polarisers) anymore. I wish I'd seen it before I trusted a trip to my "good" tiffin Vari-ND, only to find that all my wide angle shots had variations across the sky because the left and right side of the frame was polarised at a different angle (And no polariser or Vari-ND is going to prevent that - it's just physics).
I forget where I saw it, but I did see a side by side of a Pgytech filter vs a PolarPro, and I can't remember if they gave either the thumbs down, I do remember that the PGYtech had a little color cast, and was consistently a little darker than the Polar Pro. Not really a problem if you know about it and correct for it, but it was there.
There's even variations in color at each density within the same brand. (Trying to find a link to it. It was vy Shane Hurlbut), but every 0.3 in this Tiffen Comparison video I saw cut a little more IR than the last. So by the top of the range, it was very "Purple". But they had new filters out with a coating to minimise this and keep it more similar. Side by siding each of the densities, you could see them drifting further and further apart.
So, an ND might be easier to make and pretty close, but... if Skyreat was too scared to actually put their product to the test but thought they could get away with that claim, doesn't that kind of suggest that not all ND's are created equal?
I'm sure there's a way to bugger them up, or improve them, so I'm curious if DJI saved some money here or included good glass to go on the Hassleblad on their $7000 drone...