In their 2 man lineups, BI/Zion had a net rating of +2.0.
BI's net rating was +4.0, Zion's was +2.9
The Pels overall for the season had a net rating of +4.5.
Zion and BI on the court at the same time created below average outcomes for the Pels. They just don't fit well together.
Also, BI and Zion healthy has yet to be a thing for an entire season. I mean, we know what this looks like when you build around these two guys. The ceiling is the play-in and a first round exit, with one or both of them missing 20-30+ games per season.