When you Hodl you are effectively loan out the amount equivalent in FIAT that that BTC represents to the market cap, and thus others to do things with that capital [the amount maybe the amount you acquired the BTC for]. Either way it is incorrect to say that not spending BTC is not Using BTC.
Wow, thia post has been written around 6 years ago and I haven't thought from this perspective. Bitcoin is such a complex, but yet such a simple economic instrument.
Just to give an example, by holding BTC and not selling it the pressure on the market is lower, thus the price can more easily rise. This impact people who get loans using crypto as collaterals as well. They can leverage more fiat money for the same amount of coins.