Yes, they could. As soon as he landed in Russia, he was under Russian jurisdiction. There is a legal fiction that he doesn’t have to go through immigration, so he doesn’t need a visa, but Russian law still applies to him, and he can be detained for questioning without charge indefinitely under Russian law. This happens so frequently that it happened just yesterday. https://www.jpost.com/international/article-893628
Obviously, but they couldn’t have technically (important word here) held him there without the revocation
Yes, they could. As soon as he landed in Russia, he was under Russian jurisdiction. There is a legal fiction that he doesn’t have to go through immigration, so he doesn’t need a visa, but Russian law still applies to him, and he can be detained for questioning without charge indefinitely under Russian law. This happens so frequently that it happened just yesterday. https://www.jpost.com/international/article-893628