They're talking about the cooling and the boards on the new Pocket because THOSE are what negatively affect the battery life when compared to the GH5S, not the battery its actual capacity. The LP-E6N has a capacity of about 1800mAh, where the DMW-BLF19 has a battery life of about ... 1800mAh.
There is a limit to how much juice can be squeezed reliably (and affordably) out of a battery. Sony's latest FZ100 seems to be class-leading at the moment with about 2200mAh. So no, manufacturers cannot simply "use a larger capacity battery". Instead they look for ways to decrease energy use of the device itself. These are not always viable: the Pocket needs more energy- consuming cooling than a GH5S because of its much higher bitrates. Alternatively, Blackmagic didn't NEED to use more power consuming boards but they opted to do so for financial and pipeline reasons. Which to me seem like small trade offs (switching batteries takes like five seconds and I shoot in a way where this is not a problem at all), however I can see how some might find this a downside.