The citizens of the secular state happen to be part of Klal Yisrael
Only the Jewish citizens of that State are part of Klal Yisrael.
His concern is that they might end up marrying non-Jews if the incoming citizens don’t convert.
The solution, then, is for the State to not to accept incoming citizens who aren’t Jewish. Not to do a pretend-conversion and pretend they are Jewish and therefore if they marry it is somehow not an intermarriage.
As far as the non-Jews the State already granted entry, pretending they are Jewish following a pseudo-conversion where they continue to be Mechallel Shabbos the day following their mock conversion not only doesn’t fix the problem but makes it immeasurably worse in fooling some real Jews into believing those pseudo-converts are now Jewish and causing intermarriage by fellows who otherwise would never have married a non-Jew.
And the only reason he (and others) came up with this ingenuity, in the guise of religion, is due to their nationalism which is otherwise known as zionism. It was never to solve a religious problem, as it in fact makes the problem worse – and it’s a problem that was seeded by zionism in the first place – but rather to solve their nationalistic concerns.