animals can play dead, and they are not intelligent
Animals are intelligent. Their intelligence does not reach up to the level of advancement we call "sapient" -- only humans do (as far as the material beings living on Earth go). In Bionicle, SM creatures and MU Rahi are intelligent, but (generally) not sapient. Yet, again, the example of Krahka shows that some creatures can go up a gradient or spectrum of intelligence as they learn more, and this might apply to the Entity.
By the way, most animals' intelligence does reach up to the level of "sentience" (self-awareness), as Greg has pointed out. Just FTR, since there's a common misconception about that.
Or we can just state it wasn't intelligent at all during those tests.
The two options aren't mutually exclusive. The Entity could have been at any level of intelligence at the time, but simply not have been present in those samples. Since he can move around to exist in any pool/sample/collection of EP, and apparently only exists in one at a time (though I suspect he has some kind of transcendent sensory awareness of them all).
If it wasn't sentient when it forced itself to the surface of the planet, how did the EP Entitiy remember it?
I don't wanna comment here on whether it was or wasn't yet, but at whatever moment you attain self-awareness (sentience), or humanlike intelligence (sapience; not sure which you actually meant), your memories of the time prior to that should still remain.
BTW, I would actually argue that the Entity has reached some state beyond humanlike sapience, that we could not comprehend, anymore than a sentient animal could comprehend human sapience, or a germ comprehend a sentient being. Perhaps we could call it supersapience, or transcendence or enlightenment. If mem serves, the Entity has implied this.
As to what that could mean as far as how it got that way, not sure. It could argue against the idea that there was a time when fully-formed EP wasn't at its full level of mental advancement. Or, it could mean that EP, once fully formed, had the capacity to become more intelligent than any being, by attaining all the good parts it could observe from all the beings and fusing them all (much as Krahka could fuse aspects of multiple powers or forms), becoming like a hivemind. Whereas beings like an Agori had much more limited "brainspace" and could only reach sapience.
Also, since everyone on SM was aware of the history the Entity cites in that quote, there's also the possibility that he didn't gain either knowledge or sapience until later, had no idea he was being forced up like that at the time, but later learned of it from hearing them talk.