It's less calling them liars and more assuming/hoping the initial test results are a false positive.  Field tests are known for those.  You'll note the story specifically calls out that this suspect pot tested positive when field-tested.  At least I'd assume that's what the various quoted individuals are thinking.
It's considered dangerous enough to warn other first responders, based on a field test of only moderate reliability, because fentanyl can functionally act as a contact poison.