It wouldn't surprise me if the lack of inventory-based puzzles was related to the switch to a 3D engine.
To me, "mechanical puzzles" means something like the puzzles in Myst, Riven, Exile, Schizm, etc. But I suspect "mechanical puzzles" may mean something else in this game -- something that was doable with their new game engine.
The experience will be more instinctive because there will be no more inventory that characterized the genre of Point & Click system previously.
I don't believe there are many gamers that think the puzzles in Myst and Riven were more "instinctive" than the puzzles in inventory-based games. It's true that some inventory-based puzzles could be highly illogical, and solvable only by "try everything on everything." But not all inventory-based puzzles are that way, and the illogicality has nothing to do with being inventory-based. You can certainly have illogical mechanical puzzles.
I'll probably try the game if it comes to GOG or DotEMU or ShinyLoot. But I don't have much hope that the puzzles will be anything like the ones in Myst, Riven, Exile, Schizm, Obsidian, or other noteworthy games that had mechanical puzzles. I'm afraid they'll be casualized instead -- or made more like the puzzles in action/adventures and be timed or dexterity-related.