This would be no different than any spell that animates an inanimate object. Would you consider a smart phone to be sentient? Come on, Siri can be very humorous! But of course not. It merely accesses information available to it from the world around it. Talking to trees is no different. Like the example in the episode, asking if orcs went by, they leaves tracks, damage the fauna, cast aside chicken bones, etc, all impressions left on the environment. The spell presents the information through the tree, the conduit to the environment that the druid uses. The method of presentation is all part of the druids subconscious.
My opinion of how spells like that work. Of course Treants are different.
This episode gave me an idea for a different vein of druids. Normally I would think of powerful astronomers as either clerics who worships the stars or anarchists who understand them at a more meta-physical level.
However, listening to this episode it really solidified to me that there very well could be druids that worship the natural world that they (likely) can never touch - each star has a slue of planets they will never touch. Not all, but some, may have life they will never know. On a fundamental level, these druids understand that the uncountable stars in the sky, each with their own planetary systems and a tiny chance of life... but even with those odds... its out there. They know it. They feel it.
It is a super valid point, and somehow feels contradictory to how most people run it. I mean, this makes perfect sense though. A bush wasn't chatting up other bushes in its own fully fleshed out personality, and just because a spell was cast it can now do that for a PC. In the same way that the bush wasn't suddenly imbued with the ability to speak in that fully fleshed out personality for the duration of the spell.
Finding the "voice" of the plant is an extremely good point, and one I fully intend to explore at my own table the next time it comes up.