You can attach a supplemental battery pack to any phone, but many normal phones (such as every phone I've ever owned) have replaceable batteries. It seems hard to believe that if you're going to make a phone specifically touted as being modular you'd be less modular than other phones already on the market.
The original Ara prototype had a small internal battery to allow hot-swapping of the battery modules. I'm not sure if this has changed and it's now more of a standalone battery, but it used to only give you about 5 minutes of power.
One of the big criticisms of the LG G5's claims of "modularity" was that the battery couldn't be hot swapped, so it seems that some level of internal battery is necessary for modularity.
