das FAQ ist zwar nicht 100%ig offiziell und aus der vorigen edition, aber IMO das einzig sinnvolle:

Q. How do weapons/items that affect all models in base to base contact
work when held by a character (or champion) in a challenge?

A. The models engaged in the challenge are considered to be in single
combat during the challenge and may not attack or be attacked by models
outside of the challenge. Therefore the weapon / item only affects the
opposing model in the challenge (and the bearer if it would normally
affect him).
S. Direwolf FAQ Council Interpretation / Rulebook page 99 / Tomb Kings
Army Book page 36 (Destroyer of Eternities Description) / Storm of Chaos
Slayer Army List (Skavenslayer Description)

beim berserkerschwert wird es genau so erwähnt - also kann man das eigentlich 1:1 übernehmen meiner Meinung nach.