Undercities and orbital bombardment

Do undercities offer some protection from base destruction by adding
15.000 hit points to baseMaxIncoming or not?

The wiki says yes, this line from the bug report says no:
Magik - Undercities currently (Host 196) don't add to the value of the
hit points/baseMaxIncoming of the base, but they do protect colonists
currently.

Phaidros

They do provide the additional hit points, which helps the damage to
the base not get to the colonists as quickly. But, also when
determining how many colonists are killed, they reduce the amount of
colonists killed (as do raid shelters). So the base will blow up
around the colonists, but they'll be alive hiding in their bunkers.

Magik