When and how did this happen? IIRC the Brummbär is still totally capable of wiping any number of models as long as they're bunched up closely enough or have low enough health - which both the ST and AVRE should also be able to do.
You're right, it's not a hardcap, this is the patch note I vaguely had in mind:
Brummbar
The Brummbar is having its AOE adjusted to reduce its ability to wipe out full health units on the first shot, but still able to deal significant damage on misses.
• AOE distance changed from 1.25/2.5/3.75 to 0.625/1.25/6; applies to both auto-attack and bunker-buster
• AOE damage changed from 1/0.3/0.05 to 1/0.35/0.2; applies to both auto-attack and bunker-buster