Hello. I am probably missing something, but they just meant they are both on your Google account, surely. Or the tablet is tethering off the S5 if it is a wifi model. If you are returning it I would reset it in Settings, which will wipe all you have input. Make a backup of media if you need them.
You can remove your Google account first if you like, though not necessary. The LG will probably stay as one of your Google account devices, but you can hide it rather than fully delete it.
Do you have security concerns?