While I could understand how being the "effective target and the caster" for imbibing potions could create that argument that anyone with an animal companion or familiar might be able to share it as if they had cast the spell, it goes on to state that this is only for control of the spell after the potion is used. All decisions that would be made at time of casting, as well as caster level, have already been decided by the true caster, the one that created the potion, which is also explained in the DMG. I would have to say that, since there is a dichotomy of "casters" for potions, the creator is the one who counts for this purpose, as it uses his caster level and initial decisions, while the imbiber only gains caster-like control after it takes effect.