Headscratchers / Return to the Planet of the Apes

• The Apes knew of the Underdwellers existence and at least knew that they were humanoid in appearance, hence Urko and his men's plot to pose as Underdwellers in an attempt to frame them for crimes they didn't commit. This was a ruse to convince the Ape Council to declare war against the Underdwellers. They also knew that the Underdwellers were technologically advanced, given that Urko credited the astronauts' temporarily confiscated laser drill to them. It was a running mandate that if the humanoids were shown to have the power of speech, let alone intelligence, they must all be exterminated. The episode Lagoon of Peril ruled out alien visitors. So what, exactly, did Urko and Zaius think the Underdwellers were, if not advanced humanoids? And more importantly, given their close proximity to Ape City, why did Zaius and the Council tolerate the Underdwellers' very existence, regardless of what they were?