I was wondering about this, and I’m thinking maybe the light will perform much better than 1000 lumens with a single 26350 cell.
There are very few places where you can get this cell, and Fasttech is one of them. The cell they’re selling is rated at 4A (https://www.fasttech.com/products/1420/10017428/5363800-inr-26350-3-7v-2000mah-rechargeable-li-ion), but HKJ tested this cell and it was able to handle 7A continuously (Test/review of Unknown LI26350C 2000mAh (Purple)). In order to obtain ~2000 lumens you need about 6A.
So maybe Haikelite rated the 26350 cell that it can only deliver about 4A, whereas in reality it can do much better?
This would explain how Haikelite obtained the 1000 lumen figure though; I don’t see it happening how just the additional voltage sag of the 26350 cell relative to the 26650 cell is the reason the light will only output ~1000 lumens.
It’s just an idea, and it would be interesting to see actual figures.