The issue is described further in the Tweet chain: Physical property retention is more or less impossible; these people typically end up getting their phones stolen every month to 4 months. The same would be true of IDs or other paperwork that could be used to prove their identity.

They get phones from a government program. Each new phone has a new number, and due to the above challenges, it'd be challenging to port numbers and keep a consistent number.

Authy accounts are keyed to your phone number, and to set one up on a new phone you have to receive a verification call/text.