This is like saying that Germany has the right to keep the American nukes stationed on its soil if the US was to ever leaver Germany.
The soviet bombs were built, operated and guarded by a Russian department of the Russian Republic member of the Soviet union. what Ukraine signed on was a smooth repatriation of those nukes back to Russian. there is no real way Ukraine could have confiscated them even if they tried.
Russia would have never signed on to that. Their whole argument about Ukraine is the constant advancement of NATO territories towards its border.