I've seen this before. The general consensus is that the app is erroneously reporting all the data usage from all users on that particular hotspot. It's just a bug and won't affect you in any way.
Can't they just have it detect how much bandwidth is being utilized by my particular device through mac address?
No, Comcast does not measure data on that level. Their system only counts data that passes through a specific modem associated with a specific account.