Lightning Network is BTC, just off-chain. You are correct however that the technology itself can in principle also be applied to other crypto assets. This is partly done already through the taproot asset protocol where other assets like stablecoins (e.g. USDT and USDC) can and have been minted as a taproot asset in the Blockchain ecosystem. Example of that is the Speed Wallets where users can pay and receive USDT and USDC via the lightning channel. Bitcoin remains the settlement layer.