This is inspired by a question for Magento 1.
I tested on Magento 2-alpha 104 and it behaves the same as in Magento 1.
When mass updating products using the Update attributes from the product admin grid, the updated_at attribute does not change.
It is only changed when explicitly calling $product->save().
This may be a philosophical question "Should updated_at be changed when bulk updating products?".
I would say "Yes". It should be changed independent of the source of the modification.
This is applicable for all EAV entities.
If you have a different view on the issue, please share.