> If they want to actually send money to someone using bitcoin, they will have to get their balance on to the main chain

Strike payment recipients can withdraw their USD balance as BTC to any Lightning-enabled wallet (or any other wallet, for that matter) [1]:

    How to purchase bitcoin with Strike

    1. Open a Lightning wallet or Bitcoin wallet of your choice. (If
       you do not already have one, see our recommendations below)

    2. Generate a receive address or invoice for the amount of btc you
       wish to purchase.

    3. Scan with Strike and hit send. Done! You have just converted USD
       from Strike to btc in your wallet.

[1]: https://strike.me/faq/howtopurchase