3 ideas
A deceiver who states their intent in words that are literally true but ambiguous can pass any test that checks sincerity. The Kanamit really do intend to 'serve man,' so the lie detector reads them as honest. Verification tools confirm that the speaker believes what they say. They cannot confirm which meaning of a word the speaker has in mind.
Ask whether a provider's generosity is exactly what a farmer would do to raise animals well: feed them, keep them healthy, and stop them fighting. Ending hunger and war looks like altruism from inside the herd but is simply good stock management from outside. When a benefactor's gifts line up perfectly with what would make you more valuable to them, the gift alone cannot tell you whether their interest is your welfare or your yield.
Gifts that eliminate a group's capacity to resist are dangerous no matter how welcome they seem. The Kanamit field that disables explosives ends war, but it also leaves humanity unable to fight back. Once trust has been used to dismantle the means of refusal, discovering the truth comes too late to act on it.