Well, it was a crazy assumption/conclusion based on different sample sizes.
For example, this year after 40 games, the Cubs have scored 215runs. Last year after 40 games, the Cubs had scored 227 runs. So did their offense get better or worse with Bregman? And is he in any way responsible for any of it?
The answer is - roughly equal is my guess. 12 runs across 40 games might be dependent on opposing pitchers. (Or their one 16 run outburst last April.)
I can’t get team OPS+ numbers across.partial seasons, but my guess is the Cubs slowly reverted from a number like 126 after 40 games down to a 118.