Liquid Sidechain can't be placed in the same category with the Lightning Network, no?
Of course, because current sidechains like Liquid are centralized. But I think it is possible to make a decentralized one.
Decentralized sidechain or not, it still won't belong in the same category as the Lightning Network. They're fundamentally two different networks with different trade-off, and different security models. "Bitcoins" in a sidechain are tokens issued, the Bitcoins in Lightning are actual UTXOs that have not been settled in the Bitcoin blockchain yet.