I was actually talking to someone that works in advertising and for big companies this is unlikely. Pepsi for example pays a lot for the guarntee that their product ads won't appear near posts they don't want them to. Since Lemmy advertising would only be through regular posts where they have no control over this, they likely wouldn't risk the potential detriment to brand perception.

Now this can change if the potential reach of Lemmy is big enough but that size will be different for each company.