A boat that you are required to keep clear of is by definition an obstruction.

From definitions section of RRS:

"Obstruction An object that a boat could not pass without changing course substantially, if she were sailing directly towards it and one of her hull lengths from it. An object that can be safely passed on only one side and an area so
designated by the sailing instructions are also obstructions. However, a boat racing is not an obstruction to other boats unless they are required to keep clear
of her, give her room or, if rule 21 applies, avoid her."

As the inside boat PW is entitled to room to pass the obstruction in terms of rule 18.2.