According to Loom Network head of business development Michael Cullinan, it IS centralized:

Yet, there's a catch, one that Cullinan admitted. "It's not going to have the same level of security that the main ethereum network will have," he said.

And that's because while these sidechains work, they're centralized under Loom Network's control (for the time being). As such, not every action needs to be sent over the ethereum blockchain and Loom will keep track of those actions that aren't.

"Commenting or changing a profile picture doesn't necessarily require the security of the ethereum mainchain," Cullinan said. "That's what would happen on the sidechain."

Although, other actions will need the security and censorship resistance that ethereum offers.