Is graph search of shortest (optimal) path an instance of optimisation?
Although I'm not sure what you're calling 'optimisation', I can tell you that both Dijkstra's algorithm and BFS are not some form of searching a solution space. If they were, they would start with an initial solution, i.e. some path from A to B, and eventually find the shortest path  between A and B after considering multiple such paths. But this is not the case, as these algorithms both construct partial solutions before eventually getting a complete solution.

On other hand, it is true that locally, the distance to some node is at first estimated by some path and possibly later improved, so there is some similarity here. However, the incremental approach to a solution is an important aspect in these algorithms, which doesn't occur in gradient-based optimisation, as these cases usually don't even have a concept of a partial solution which can be extended into a full solution.