It is possible that DeWalt designs and manufactures its own batteries, and there is some clever tech in them that makes them superior to other batteries, or that they pay for high quality components that make them last longer.

However, it is just as likely that they buy the batteries themselves “off the shelf”, put them in a custom plastic enclosure, slap their label on them, and sell them for an insane markup. Sometimes, they don’t even need the custom enclosure and they just buy someone else’s finished battery product. The “off-brand” option may be the exact same product.

It’s not unethical to use off-brand batteries, but then if you buy a substandard one that causes a problem the manufacturer may use that as an excuse to deny their warranty claim. But when was the last time you replaced a tool under warranty?