I wouldn’t say “treason” but they are most certainly bordering on, if not actually, a chillul Hashem. There’s nothing wrong with being opposed to Zionism. There’s probably nothing wrong with opposition to Israel as a Jewish state. There is a huge problem to publicly announce that Jews support the “Palestinian cause” as it’s essentially support for anti-Jewish terrorism.
Avi K: Only according to some versions of religious Zionism which was viewed as nonesense by the vast majority of frum Rabbonim 60 years ago and is still viewed as such by most gedolim.