The Cubs' defense is phenomenal, probably cutting ~0.50 or so of ERA of the pitching staff's ledger. WAR is correctly giving that credit to Cub hitters not pitchers.
People get snippy about "luck" but broadly the Cubs have been very unlucky in terms of giving up homeruns. Probably to the tune of ~0.4 runs a game. Pitching WAR does not at all attribute that to luck, it completely counts against them.
The Cubs have broadly been getting lucky interms of sequencing to the tune of about ~0.2 runs per game. What I mean by sequencing is if a pitcher gives up a homer and two singles in an inning, whether that inning results in 1, 2, or 3 runs. Fangraphs pitching WAR treats this entirely as luck, but I think baseball reference might give the pitchers a little credit?
So all told the Cubs have been average in terms of run prevention, and how that translates into WAR is frankly an accounting exercise.