Err... you're not wrong. My initial comment would have made more sense if this had been the post I was intending to reply to. Not sure how I managed to screw THAT up.
Long story short if I remember the video well after watching it a while ago: while sand is cheap for thermal energy storage, storing it with reasonable efficiency requires a massive unit (much higher capacity than one home would reasonably need; can't be scaled down well) held underground (immediately incurring massive costs).
They're saying that storing it underground could mitigate the size issues. Though I query if we'd need these if we simply installed ground source heat pumps everywhere?