I think they're building eth-style rollups on top of rootstock, which is an eth clone that is also a federated bitcoin sidechain

So it doesn't currently have anything to do with bitvm, but some of the rootstock guys have expressed interest in bitvm. If it really offers an improvement over their current federated model I wouldn't be surprised if they rebuild rootstock's deposit/withdrawal mechanism to take advantage of bitvm's features. Let's wait and see. Bitvm doesn't have any working sidechain code yet but myself and some others are working on it.