It’s not the logarithm of the BTC price that is a martingale, it’s the BTC price itself, under a risk-neutral measure if that makes you more comfortable (since BTC derivatives would be priced by the same risk-neutral measure pricing BTC itself).
It’s not the logarithm of the BTC price that is a martingale, it’s the BTC price itself, under a risk-neutral measure if that makes you more comfortable (since BTC derivatives would be priced by the same risk-neutral measure pricing BTC itself).