Citizenship Act and the Reciprocity Proviso

Subramaniam Swamy has supposedly dug out a legal provision under which Sonia Gandhi cannot become the Prime Minister of India.

The provision he is referring to is under Section 5 of the Citizenship Act. Unfortunately, the Internet does not seem to be a good resource while excavating such legal/constitutional information concerning India.

So far this is the only place where I could find text related to what Swamy is referring to, and I quote -

Citizenship by registration
(1) Subject to the provisions of this section and such conditions and restrictions as may be prescribed, the prescribed authority may, on application made in this behalf, register as a citizen of India any person who is not already such citizen by virtue of the Constitution or by virtue of any of the other provisions of this Act and belongs to any of the following categories,-

(a) persons of Indian origin who are ordinarily resident in India and have been resident for five years immediately before making an application for registration;

(b) persons of Indian origin who are ordinarily resident in any country or place outside undivided India;

(c) persons who are, or have been, married to citizens of India and are ordinarily resident in India and have been so resident for five years immediately before making an application for registration.

(d) minor children of persons who are citizens of India; and

(e) persons of full age and capacity who are citizens of a country specified in Schedule I:

PROVIDED that in prescribing the conditions and restrictions subject to which persons of any such country may be registered as citizens of India under this clause, the Central Government shall have due regard to the conditions subject to which citizens of India may, by law or practice of that country, become citizens of that country by registration.

Legal provisions are not supposed to be understood easily and this I think is a law in itself. As is the case in the above law. The last sentence beginning "provided" is so convoluted that it took 3 or 4 readings to even guess what it could possibly mean.

And it does look like the law can be interpreted in a way Swamy is talking about. But I am making a few assumptions here.

1. That this website is authentic
2. That this is the only law that talks about this - and that there is no other law that gives a more clear interpretation that overrules this one.
3. That my interpretation is logically alright

Can someone throw more light vis-a-vis my assumptions?