Possibly, I personally think the reason is leverage. They have something we want, now they have something we want, hence suggestion of the complex hostage exchange back in Apr 2019 by Iran’s foreign minister with respect to Nazanin Zaghari-Ratcliffe. BoJo's comments did not change anything. They held her for that long prior to BoJo's comments and would have done so until they had something they wanted, regardless of Johnson's comments. They are happy to build up ammunition (hostages and leverage) and use it in turn when they want something.
Will play out broadly similar here regardless of if there were British citizens on the tanker or not I expect, will likely be Grace 1 for Stena Impero