Sure, aircraft can be more efficient at higher speeds, but then when speeds pass a certain threshold, drag starts to dominate and efficiency drops like a rock. There's a sweet spot, and it's certainly less than the plane's top speed.
It's like if someone said, "going a little slower on the highway [typically ~60 mph+] saves on fuel" and someone pedantically ButAkshully'd that cars are more efficient at 40 mph than 30 mph because of engine performance.
Edit: Ironically, I'm the one normally promoting my "Scylla-Charybdis heuristic", that you should be able to identify when tweaking a parameter goes too far. But here I think the context of the tweak was clear enough that this caveat was a distraction from the core point.