The theory is because Bitcoin's supply is fixed, it must go up when fiat goes down. But the reality is not so simple, there are many factors that determine Bitcoin's price, and fixed supply is only one of them.
As far as I see this for me the theory is simple. People have more money to invest they put more money in Bitcoin, people have less money to invest or in the extreme case, they run out of it they no longer buy coins but sell them. Inflation just accelerates things, everyone was saying in an economic crisis with an incoming inflation rush for real estate, I've seen with my one eye how this did NOT work a decade ago with everything crashing around.
Yeah, everyone says to buy when there is blood on the streets but what happens when that blood is everywhere because everyone is dead? Easy to say cheap coins, let's buy but with a lot of people afraid of what happens next month it's for many cheap coins or food.
Can you pointed out to me where did Satoshi said Bitcoin was created to hedge against inflation?
Satoshi didn't say a lot of things about bitcoin but people like to fantasize about what it was created for, in reality, it was just a p2p currency, ironically enough, almost everyone ignores the actual purpose it was built for and seeks to reinvent it by adding a ton of things no currency will be able to solve.
But it's understandable, a lot want to be millionaires by holding 100 satoshis, you've got to market the shit out of it.