There are many other letters showing that all gedolei Yisrael recognized that “Religious Zionism” is idolatry and heresy, not just the prior Chabad Rebbe.
It’s not a secret and it’s rather obvious, too. The “Religious” Zionists tend to be the most fanatical Zionists of all Zionists.
Chabad is a significant movement in the Zionist “State”, so they need to be in the good graces of the wicked Zionists.