He's not blaming Sanji, just pointing out how Oda has been handling the SHs post time skip. If you're looking for some focus on Usopp as a character and for him to naturally grow both from character development point of view and fighting abilities in a natural way it likely won't happen.
@kkck is right. Aside from Luffy none of the SHs (Sanji included) have gotten much development post time skip. Feels like Oda has a checklist of where he needs the characters to be from a narrative/strength point of view and just jumps everything in between. Regarding your original question Usopp will be shown to be strong enough to defeat characters on the level of Oven, Page One, Izou, Daifuki, Sasaki etc since the plot demands that of him. So you will get something but it will be forced by plot since the Usopp we have seen for the last 15 years by current power level standards is weak. But Usopp will somehow end up defeating Van Auger a powerful opponent by end of story without any actual fights in between where we get to see him develop skills.
As an example Usopp activated observation haki when taking out Sugar in chapter 758, which was released in 2014! It has been almost 11 years now and we have not seen Usopp use haki at all. Unfortunately, this is all we will get from Oda on a lot of the original Strawhats for the rest of the story since he's gotten too busy with other characters. They're getting the same treatment as a lot of the original characters (Krillin, Yamacha, Tien, etc) in Dragon Ball.