quoting your own link: ... However, Max von Laue quickly rebutted those claims by saying that the inertia of electromagnetic energy was long known before Hasen

quoting your own link: ... However, Max von Laue quickly rebutted those claims by saying that the inertia of electromagnetic energy was long known before Hasenöhrl, especially by the works of Henri Poincaré (1900) and Max Abraham (1902), while Hasenöhrl only used their results for his calculation on cavity radiation. Laue continued by saying that credit for establishing the inertia of all forms of energy (the real mass-energy equivalence) goes to Einstein, who was also the first to understand the deep implications of that equivalence in relation to relativity.