So either a Mod made a mistake, or Bitcoin LN is considered an altcoin on Bitcointalk.
The issue (from the mods PoV) could be less about LN being directly related to Bitcoin, and more about the altcoin involved in the trade.
I'm saying this because you want to sell altcoins. Fair enough, you want btc for them but the idea of the thread is the sale of some alts.
I see your point, but the rule includes a part about the thread belonging there if it involves Bitcoin. Also, the summary rule for that board is;
Buying/selling bitcoins using other currencies. Except, other currencies is limited to only fiat, a trade involving Bitcoin and an altcoin is fitting there.