Hi thangCQ,
From looking into this it does seem that you’re right that the productReorderMedia mutation isn’t supported in bulk operations for setting an image as the first in the list, which means you can’t use it to reorder images in bulk.
This is indeed a limitation if you’re looking to update multiple products at once. If you’re handling a large number of products, you might consider automating these individual calls through a script or app that processes them sequentially. I’ve also flagged this as a feature request to the product team that owns this space.