No, topics are much more than tags which doesn't mean that topics can't be used for tagging purposes with a "tag topic" and an appropriate association between the topic that is being tagged and the topic that represents the tag - hope that makes sense).
Topic maps can be used for meta-modelling purposes: to model other models. So, a project is a topic (of type "project" - the type being a topic itself), a feature would be a topic, a feature set would be another topic, milestones, stakeholders, basically everything would be a topic with the necessary associations to express the appropriate relationship between the relevant topics.
So, in your case, feature topics would be aggregated together into a feature set by asserting the necessary relationships between the individual feature topics and the feature set topic. Once that is done, a set of milestone topics could be created with each milestone separately connected to the relevant feature set topic(s). You could then add specific date-specific metadata (key-value pairs) to the milestone topics so that they could be navigated in a timeline-like fashion.
Let me know if the above makes sense or if you would like a more in-depth explanation.