Exactly how was Trunks supposed to get any real stronger, when he didn't have a gravity machine, sparring partners, anything?
He most likely struggled to survive with his mother throughout that entire time.
More importantly, there are exactly ZERO statements indicating such a huge power up.
As indicated by Goku's quote before he uses the RoSaT, training would do him no good.
The humans were completely clueless on the power of a Super Saiyan, they didn't even see Trunks and Goku in proper action and were just banking on Goku being a Super Saiyan and thus beating the androids.
That's when Krillin even says after Goku is beaten.
I fail to see, how that argument is nonsense.
Training in an harsh conditions won't make him any stronger, unless he ascends.
So how is a couple of years spent training in much less harsher supposed to make him much stronger?