I think there's a little bit more too it. In this situation, it gets brought up multiple times how easily Trunks dispatches of Freeza, using his power as some sort of standard. Freeza is literally nothing to Trunks, even with the help of his father he is a non threat. Vegeta is a overly cocky character, Trunks on the other hand, is not. He is just so confident in his overwhelming power advantage, he can't help but boast.
Freeza and Goku on Namek both pale in comparison to Future Gohan and Future Trunks. There is a wide divide there.