They had plenty of ex PARC developers in their team and even a ST80 license, so they could have done so no doubt. Obviously they considered ST80 not mature or efficient enough for their projects and invented Object Pascal instead. This is actually not that surprising considering that neither Xerox used ST80 in their products. Even the first WYSIWYG and DTP programs in existence were written in BCPL (precursor of C) on the Alto, not in ST (see http://xeroxalto.computerhistory.org/xerox_alto_file_system_...). There is an article by Tesler where he describes how they had to increase the frame rate of a demonstration video ten times because the application written in ST76 was so slow (see https://dl.acm.org/doi/10.1145/2212877.2212896).