It derives its functionality and value from an onchain tx. How can it not be a derivative? Unbroadcasted is the keyword here. Maybe it's a philosophical question to have. But without finality that an onchain transaction only guarantees through accumulated proof of work you theoretically have nothing more than a signed promise that if everything works as intended should represent a BTC transaction in the future. The risk that something fails may be < 0.000001%. But it will never be zero. Take for example the Qubic scenario with Monero. Chances are it would make a mess out of LN transactions and I wouldn't call this a long tail risk.