It's just their way of boosting initial sales. Afterwards it will likely be available from their regular shop and retailers.

As a tiny, relatively insignificant corner of the global supply chain, us flashlight nerds will get whatever is being cranked out by the bucketload for other industries. We got Li-Ion in 18650 form factors originally so readily because every laptop on Earth was stuffed with them for a while and they became a high volume commodity product. Now, BEV cars are being packed full of LiFePo cells which will undoubtedly become the new commodity standard until the next thing comes along.

There will always be enthusiasts paying top dollar for whatever is mathematically "the best," but products for ordinary consumers will by and large be designed around whatever is cheaply and readily available to the manufacturer. Jury's out on whether that's a good thing or not. LiFePo has a lower wattage density, but should be able to tolerate more charge cycles which is kind of the point. I guess that'll be attractive for buyers of whatever stupid trendy non-replaceable-battery gizmo goes on sale at REI next week...

I didn’t see the weight of the battery mentioned so can’t compute WH/kg, but I didn’t look closely.

Yeah, it's mentioned right at the beginning of the review:

Weight (38121 battery): 342 g