Even a capital-backed idealistic ideology supports an idealist argument? That's like saying even Marx criticised capitalism.
There's a moral argument in favour of these boycotts, but that's all it is. Even if one holds comrades to such moral standards, and I'd agree with that, it's still not a real version of combating capital. Idealists would have us believe the political economy is simple enough that a fraction of a fraction (people who hold to a boycott, among people informed enough to know about the wrongdoing sufficiently) of their consumer base refraining can compel companies.
Snowflake in an avalanche type of arguments only hold up from high potential, in a manner of speaking changes that are inevitable anyway. Improving conditions against capital requires building up potential, uphill. This can be done by organising and educating, not as an [expected] accumulation of individual actions and browbeating people for not engaging in just that.