it does demonstrate (to me, at least) that if we gave this test to adults, as the OP suggests, we would not be testing at all the same things as if we gave it to children
You would if you used the same conditions though LRD. The adult is taught, every day, over a year and half, how to pronounce each grapheme.
They are taught to 'sound out' dog, fog, sog. It makes no difference is the word is 'real' or not.
They are asked to read the 'sound' and say the 'sound': d-o-g, f-o-g, s-o-g.
Adults would have no problem with this. If the grapheme has two or more plausible pronunciation, either is acceptable.
The nonsense word 'spown' could be pronounced with the ow as in brown, or the ow as in throw.
Either would be a pass because you would be using a correct pronunciation of a grapheme.
Phonics is very easy. Blending is harder and that's why it's so important to learn the pure sounds.
Otherwise sounding out of dog would be duh-o-guh (du-ogger).