Yeah it's still detention for the purposes of the ECHR, and you can still apply for a writ of habeas corpus. That's why they need to act fast as each detainee will get legal advice and seek an injunction against removal whilst the constitutional legality of the Rwanda Act is tested. Sunak has said he'll ignore any injunctions granted by the ECtHR, he can't ignore injunctions from our own courts although they will expedite hearings. Expect to hear from the SC some time this fumner if parliament can actually declare a third country safe as a matter of fact - but if they say that's bollocks they were warned by absolutely fucking everyone. And if the govt ignores an ECtHR injunction and someone gets removed to Rwanda and that's later deemed unlawful by the ECtHR they'll just require the UK to bring them back. There was a good explainer on the bbc website earlier in the week.
In the meantime assume emergency applications to the courts here asking them to find removal is unlawful for domestic legal reasons and nothing to do with ECHR rights which should be easy. They are just gearing up for a stand off with the courts.
Current third PM since last election starting a fight over a policy not in the manifesto and trying to paint other branches of govt that stop him or delay him doing this as enemies of the people. It will play well to some Tory members but that's it. .