The synthesis of atomically precise carbon nanostructures in ultra-high vacuum has seen extensive progress on metal surfaces. However, this remains challenging on chemically inert surfaces. It is because the thermally activated C-C coupling encounters a severe "desorption problem" on weakly interacting substrates. In this study, we report an extraordinary [2+2]+[2+2] cycloaddition triggered by mild annealing (~210°C) in a highly ordered π-conjugated molecular array on graphite using scanning tunneling microscopy. In contrast to irregular dendritic fragments typically obtained on metal substrates, large supramolecular islands are observed here with cycloaddition products and other polymers over 30 %, which are embedded as defective individuals or chains (grain boundaries). First-principles calculations reveal that the energy barriers of the multiple dehydrohalogenation and cycloaddition reactions are reduced by catalytic Fe atoms but remain energetically unfavorable. A distinct driving mechanism is proposed for redirecting the reactions on graphite surface, where additional intermolecular coupling, steric hindrance, and interfacial interactions play significant roles. This study introduces a new paradigm for understanding on-surface synthesis on non-metal substrates.