For one, they don't have to be different. "How are they different" could indicate an incomplete understanding of why tools are written. Sometimes tools are rewritten just to erase some traumatic memories from the developer's subjective coding experience! (Not a comment about Java!)
Second, I wonder if this developer is stuck in the big-picture fallacy--in which calling two things "bundlers" means they're the same. Ignoring the relevant but yet-functionally-unpassed argument-details. This would also be relevant.
I remember getting caught in such a discussion once with an author of graphics tools. They couldn't understand how two "graphics softwares that did X" were really that different?
But it was like asking how PhotoShop and Affinity Designer could possibly both need to exist--details really do matter.