Thanks for the explanation! I was curious how the targeting differed between the two mAbs and found a paper that explains:

Specifically, lecanemab mainly targets Aβ protofibrils, while aducanumab and other monoclonal antibodies favour highly aggregated forms of Aβ [5]. This differing target profile may also explain a substantially lower incidence of amyloid related imaging abnormalities, such as transient immunotherapy-related brain oedema and microbleeds, with lecanemab.

In other words, lecanemab seems to bind and remove Aβ before it aggregates, while other therapies (aducanumab) bind and then remove it after aggregation occurs.