"Technical diving" is not a well defined and widely agreed upon concept and, therefore, can differ considerably in comprehensiveness depending on who uses the term.
For me, technical diving specialties have in common the fact that they deal with techniques designed to mitigate the risk associated with the impossibility of immediate access to atmospheric air. The training offered nowadays to qualify a person to do these dives emphasizes self reliance (as a solo diver course) but also improved basic skills (buoyancy, trim) and the use of a robust system of redundant gas supply (manifolded doubles, independent sidemount, rebreather). More often than not, the training also deals with improved cooperation among buddies / team members.
For me, then, it is incovinient to group solo diving with more commonly accepted technical diving specialties because it does not share those foundations that the other specialties (deco, wreck, cave) do.