I don't think Trunks was considering the power of Black's SSJ transformations there, tbh. He was likely referencing his fight with Black that took place just before he fled back in time to recruit Goku and Vegeta -- wherein base Black stomped him without much effort at all:
https://i.imgur.com/6qC2k31.png
https://i.imgur.com/3jpODzm.png
https://i.imgur.com/69FdEzU.png
https://i.imgur.com/eq6NXwT.png
https://i.imgur.com/hiCd8zC.png
https://i.imgur.com/SUJDKWt.png
https://i.imgur.com/mOGeAlO.png
https://i.imgur.com/jJOld0y.png
https://i.imgur.com/AKhR4Id.png
https://i.imgur.com/kUxwh0v.png
*Mind you, later in the same issue is when Trunks sparred with Goku, and made this comment about Black's power:
srug
