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.
Some big exchanges have been contributing to the total amount of BTC going around in the market and they are the ones who usually make the liquidation to make BTC the biggest core of the crypto market. If they also hodl their BTC, then this whole thing will be nothing and the price will not continue to grow. The holders are also contributing to this situation and the best part of it is when the price continues growing, the supply of BTC continues to decrease in the market and that will result in to rise in the price even more.