It's a little bit of a stretch, but hear me out.
The Zhat Vash's existential fear is not of synthetic life per se, but of a specific kind of synthetic evolution — the kind that arises when organic minds create synthetic minds in their own image, especially if those synths begin to self-improve, replicate, and approach or surpass organic consciousness.
This fear is derived from the Admonition, a warning left by an ancient species that had experienced an AI singularity, culminating in the summoning of a cosmic-level threat (e.g., the extra-galactic AI civilization teased in Picard Season 1). According to the Admonition, once a society reaches a critical threshold in AI development, it sends out a signal that brings these synthetic destroyers.
Thus, the Zhat Vash’s primary objective is preventing that threshold from being crossed.
In Star Trek: Picard, it's only when synths like Soji and Dahj—made from Data’s legacy but enhanced via modern methods and biological integration—begin to pass as human, evolve emotionally, and unlock ancient AI knowledge that the Zhat Vash panic and intervene.
Data and Lore, however, were handcrafted, and fully mechanical androids. Their brains were not modeled on any specific human neural pattern. That's why the Zhat Vash could have regarded Data and Lore as "controllable anomalies".