Its libertarian because it is fundamentally about individuals acting on their own without going through the government, ect. It is an individualistic framework. Individuals going about achieving their goals, even through powerful corporations, falls squarely within what libertarians support.

Yes, you can make some philosophical point about how if corporations are powerful enough, how is that in any way different from governments.

But, powerful corporations controlling society, in some sort of fallout style or bioshock style dystopia clearly describes a libertarian dystopia, not a left leaning or even fascist dystopia.