I never said it was impossible, but it being highly unlikely is a fact especially at your age. If your bodyfat measurement was accurate (different methods vary) then you are a genetic god, without question.
My measurements are accurate to the best of my knowledge. A physical therapist taught me how to use calipers, and when she instructed me in this skill, she measured my BF% along with me, and that was when I weighed 240#. Each time that she checked that I measured myself accurately, I had a BF% of 8 or 9 %. .
I never said anything about not making gains, but you weigh more than Arnold ever did even in his "soft off season state"
I have good genetics, I guess. To illustrate, my father is a small man who weighs 170# and was born approximately 2+ months prematurely. I have seen him toss a bale of hay with one hand/arm over over ten feet. This explains why, at least for me, why I have always had good strength and proportion for my size.
Hence my comment, you are either very tall OR you are a monster.
I am 6 ft tall, and I do not consider myself a monster.
I am about 50 pounds lighter than you at the moment, and I have a 51 inch chest. If I were nearly 290 with approximately your bodyfat levels I can calculate out that I'd have 21.25-21.5" arms and about a 57 inch chest, and I'm 6'2".
My stats do not measue up to yours; I guess my size is more evenly distributed.
A guy I work with had drop foot after a car accident but an operation fixed him right up.