I think they are referring to Netanyahu being of secular Jewish upbringing. This is a philosophy that puts the religousity of Jewishness on the background and focuses more on the cultural and historical aspects.

Secular Jewishness is quite entangled with the Israeli state and is compatible with Zionism.

When questioned about it I would not expect him to say he's an atheist, though, but he doesn't openly deny or affirm his faith. He said he believes in god, but it's not religious.

Citation : https://religionsfacts.com/does-benjamin-netanyahus-branch-of-judaism-matter/