I agree it is specified activate, but it might be intent of rules that it is implied that already active can perform removal - 4.2.1 3rd bullet.
There is no such implication, much less intent. Read the text in 4.2.1 3rd bullet again:
"Activate 1 Underground Irregular or Ranger there to remove 2
enemy pieces. Bases may only be removed once no other enemy
pieces are there. Tunneled Bases (1.4.4) may not be removed
(Underground Guerrillas may)."
The rule explicitly states that the Irregular or Ranger you Activate to use this bullet must be Underground.
Raid and transport have nothing to do with advise, where does it mention raid or transport?
Transport turns Rangers Underground so you can use them with Advise or Raid.