This is semantic, but probably important:

A) Bitcoin (UTXOs) *cannot* be stored offline

B) Bitcoin (UTXOs) exists on The Bitcoin *Network* (requires network connection)

C) Keys can be stored offline. Keys are not Bitcoin. Keys prove ownership of Bitcoin (UTXOs)

For you to spend Bitcoin you need two things

- Keys

- The Bitcoin Network

Without a network your keys become useless. Whether or not they are "offline" doesn't matter.