its not an altcoin, its a claim in a channel on a the last satoshi. Whichever side of the channel has more millisatoshis gets one satoshi and the other side doesn't. The channel will always either have no millisatoshis on either side, or a sum of 1000 millisatoshis split between the two ends of the channel.

Brunswick

bitcoin

Lightning network uses mSats - is that an altcoin?

I would consider millisatoshis (msats) to be an altcoin because they never appear in a Bitcoin transaction. The msat balance between Lightning nodes is just a promise, it's not enforced by smart contract.

Sorry for not answering the OP's actual question!