Um no, you deposit all your bitcoin into Coinbase's bitcoin wallet so there's no crypto enforcement that you own the bitcoin anymore. That's the only way Coinbase can safely use your bitcoin on their lightning network connections.

They need to commit regular bitcoin into a lightning network channel on the main blockchain before they can spend it.

That's not lightning. That's just how exchanges work now.

That is how exchanges have always worked, but the lightning network encourages people to use centralized exchanges that already have lightning network channels between all other exchanges and sellers so they don't have to waste time committing unknown amounts of bitcoins between 100+ vendors.