Moreover, with lawnmowers, some weight is a good thing.
There are some things where people associate weight with quality(expensive watches, bar glasses, pens, etc). A low weight lawnmower may be associated with being cheap and flimsy. So having the battery weigh more is actually a good thing for the lawnmower. There's probably an optimal point where you want the lawnmower to weigh enough so it feels sturdy (and stays where you put it), but not so heavy that an average person finds it unwieldy.
In contrast, with a consumer electronic device where lighter is always better and any weight from the battery is a bad thing. So the manufacturer has to spend more money to try to get the battery to way less.
I think the spokesperson from Sony showed Mr. Ockenden a lot of patience in actually answering his question, rather than just calling him an idiot for comparing laptop and lawnmower batteries, which was fully within his rights to do.