It may be partially due to defective hardware (or software, seems like something's stuck processing/downloading).Which is pretty obvious in itself. A larger battery is going to give better battery life. If anyone is expecting an SE to give better battery life over the XR, 11 or 11 Pro models then quite frankly they're stupid.
Besides, that isn't the target audience of the device, like you said it's the people with the older devices who are most likely to upgrade to the new SE.
My comment was simply based on the poster saying battery life on the SE was "terrible", when they had a faulty unit so shouldn't be stating that SE's in general have bad battery life when they don't, theirs did fair enough - but that's not the popular opinion.
However, poster was coming from the 11 Pro Max, afaik. That's bound to color expectations although it really is unrealistic to expect the iPhone SE with 1821 mAh battery to come anywhere near close to the 11 Pro Max with 3969 mAh battery. That's potentially half the battery life right there. Maybe more if using dark mode, etc. despite the larger resolution and screen size.
The SE is more for light to moderate users. It's not for users who spend a lot of time on their phones.