It is not like the J-10Cs would go into the trash. They can reassign them to one of the remaining J-7 units. Or a unit with J-10A.
No one is suggesting the J-10Cs would go into the trash.
Rather, it is observing that a relatively recent upgrade to J-10Cs, then subsequently supposedly to J-20s, seems somewhat odd, given new fighter procurement would usually be done in a manner to minimize wastage of time and resources (both needed to retrain pilots and crew, as well as to fit an airbase to accommodate a J-10C versus a J-20).
That leads to the very valid question of whether the 5th brigade really has received J-20s or not -- and if it ends up being that they had received J-20s, then why did 5th brigade receive J-20s rather than some other brigades that use older fighters that would make more sense to be re-equipped with J-20s from a time and resources point of view.