This is really interesting, I just read Harry Long's book, it's really short. He calls his concept "structural arbitrage," it is basically an arbitrage between two common equity hedges, short term VIX volatility futures, and long term bonds.
He reasons that if VIX futures are related to equity markets and long term bonds are related to equity markets, then VIX futures and long term bonds must also be related to each other. VIX ETPs and long term bonds are both used as insurance to hedge equity risk, but the former is usually more expensive than the latter, so the strategy is to sell the expensive insurance, and fully re-insure the risk you have taken on by buying the cheaper insurance in the long bond market.
With this fundamental concept in mind, it looks like you don't need to find any assets to short. For example, XIV is short volatility, and TMF is 3x long on 20+ year bonds, buying both assets will achieve the same goal. Shorting the inverse assets is a way to get more bang for your buck, but the strategy does not depend on it.
Thanks for sharing this, you have likely sent me down quite the rabbit hole.