The Flash has faced a variety of metahuman villains over the course of its nine seasons. However, a frequent complaint from fans has been that the show relies too heavily on speedster villains.
For example, Zoom disguised himself as Harrison Wells in order to infiltrate Team Flash, while Savitar used his speed to create a duplicate of himself in order to trick Barry.
The Reverse-Flash's observation was a clear nod to the fans who have been complaining about the show's reliance on speedster villains.
It was also a way for the show to acknowledge its own flaws and to try to make things right. By acknowledging the complaint, The Flash was able to show that it was aware of the issue and that it was taking steps to address it.