Rootstock is **not** a Bitcoin sidechain as per the sidechains whitepaper. It does **not** benefit from Bitcoin hashrate.
That is the definition of a sidechain, a separate chain that has its own mining, own blockchain, own network and the only link it has to bitcoin is usually a 2 way peg. It isn't really using bitcoin or its blockchain to benefit from its hashrate. It just benefits from its name!
RSK is a Bitcoin sidechain, so it has its own network, and its own blockchain, but not its own token.