They probably fit a few adults but the odds that everyone on here is that skinny is zilch. I have seen a few guys in the range of about 18-25 years old, so technically adults, that have a waist of about 26" inches, but that is likely less than 1% of guys that age.
Regardless to correctly fit in Huggies Pull-Ups you would need a waist below 23 inches, and I have never seen anyone over 18 without medical conditions with that small of a waist or hips for that matter. And for the S/M Goodnites they go up to size 8 or a 24 inch waist roughly granted they do stretch but then they would not be fitting as intended. But it is conceivable for a few people on here to be skinny enough to fit correctly in the L/XL Goodnites as they go up to size 14 which for boys is usually a 27 or occasionally a 28 inch waist. I myself have around a 27 or 28 inch waist, I weigh about 130 lbs, I am 6'1" and I am over 18, and honestly the L/XL Goodnites barely fit me as intended, but most people on here likely have a waist of at least 30 inches if not more. And ladies hips usually are their limiting factor, it actually seems that ladies actually can't have as skinny of waist and hips as an adult as guys can, but it evens out a bit with our anatomy so to speak.
I know the point you are making though anyways that 99% of the people on here can't fit correctly (as the manufacturer intended) into Goodnites and none of us unless someone lied about their age can fit correctly into Huggies Pull-Ups.