Nope! It applies to persons, it is not limited to "natural persons" (probably due to an oversight) which is why we have issues with money = speech. It is also not limited to citizens and previous court rulings have upheld access to right protections for "people" or "persons" to non-citizens and even non-residents, whether these rights extend to persons not within the US is more hazy but most rulings have shown that US government entities must not act to violate rights afforded by the constitution/bill of rights to persons overseas... sort of like it was an overseas code of conduct.
The person vs. natural persons issue is irrelevant to the Citizens United decision.
Free speech being extended to corporations has nothing to do with the legal concept of corporate personhood. Read the decision, it was very explicit. The rights of the people who comprise a group were held to extend to collective action by that group.
