picklescan before 0.0.21 does not treat pip as an unsafe global. An attacker could craft a malicious model that uses Pickle to pull in a malicious PyPI package (hosted, for example, on pypi.org or GitHub) via pip.main(). Because pip is not a restricted global, the model, when scanned with picklescan, would pass security checks and appear to be safe, when it could instead prove to be problematic.
The product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are not allowed by policy or otherwise require other action to neutralize before additional processing takes place, but the list is incomplete, leading to resultant weaknesses.