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Okay, after being an idiot and not watching the video to the end, I now actually drilled the 4mm holes which leads to the PLA connectors sliding into position much easier.

I did try an TPU connector anyway, and there's a point to them: They are a little less strong in keeping the cubes together (ie: less force required to pull cubes apart), but in testing here a few PLA connectors broke apart in the process. The TPU connectors are still in good shape after pulling two cubes apart about 20 times.