Not very advanced mathematics at all.
The conclusion is that if this were a rocket it'd be nearly a MILE WIDE ~ which is improbable. Therefore, the analysis suggesting this to be an airplane related event is probably right.
You calculated that on the presumption that it was a vertical rocket plume in a relatively close position right? At a maximum about 100 miles away?
If it were a contrail of an airplane at 30k feet and the far end of it appeared to come down to the horizon because of perspective wouldn't that far end be much much further away than 100 miles? Wouldn't that also mean that a given width in the camera frame would calculate out to be much much wider than the same width in the frame of an object only 35-100 miles away?
I don't think I have ever seen an airplane contrail stretch out a mile wide, much less far wider than that, and not disappear entirely. By the time they get a few hundred feet wide they get pretty thin and wispy.