Here's the response I got from the product team:
MS15-091 is covered under CSWU-003(Q3081436): Cumulative update for Windows 10: August 11, 2015
If you look at the KB (https://support.microsoft.com/en-us/kb/3081436), you will see one patch covers 6 bulletins. If we added the same patch under each bulletin it would show missing 6 times. We chose to add it under a bulletin for Cumulative Security Update.
They also noted that it's unclear yet as to whether or not MS will consider Edge a separate product. If they don't ship it for Windows 7 or 8.x then they'll likely treat it as a Windows 10 feature and Symantec will consider Edge updates to be included in the Windows 10 product.