In the DOP for AI masks only the mask general description, not the full mask pixel level. And as i see, for all mask is a similar manner.
Example: AI_MASK_02_01 described in DOP file (not my best formating, but i think its visible enough):
So, its in the DOP its don’t has pixel level info.
Its save the AI mask basics (like enabled or not, etc.) AI Prompt, etc.
Example for Brush, works in similar manner:
So, its seems recalculated in export, or at least its recalculated, or some magic happen.
What you suggest, its may works if its flatten all mask to one. However, i’m pretty sure that’s happen (flattening) before export, or at least in the very end of export. But its calculate.