He knows that there is an understanding that zionism is not judaism which is why he has to specify the contrary. As others have noted:

1. Zionism is not Judaism.
2. There are multitudes more non-Jewish zionists than there are Jewish ones.
3. To the Zionist the only valid Jewish identity is that of the fascist settler-colonialist. They seek to erase other Jewish identities especially anti-zionists ones; zionists seek to normalise colonialism, ethnic cleansing and genocide. Zionists often slur Jewish peoples that oppose zionism as "self-loathing".
4. Number 3 is a common imperialist/capitalist phenomenon. See for example the erasure of communist Ukrainian identities in favour of fascist diaspora.
5. Zionism is deeply anti-semitic.