Inflation is not price level. It is increase in the money supply. An increasing price level is the result of inflation. By definition gold is inflationary since its supply continues to increase.
Next time you attempt to discuss economics with somebody I suggest you educate yourself a little bit beforehand, to at least get a grasp of the basic concepts. The Core project has published an introductory textbook [1] that you may find useful for this purpose (although any book will do, really). In particular, chapter 13 deals with inflation among other topics [2]: "Inflation is an increase in the general price level in the economy, usually measured over a year."
Before suggesting somebody read an introduction to economics to learn about inflation, please try to take a deeper look into what inflation really is:
> Inflation, as this term was always used everywhere and especially in this country, means increasing the quantity of money and bank notes in circulation and the quantity of bank deposits subject to check. But people today use the term `inflation' to refer to the phenomenon that is an inevitable consequence of inflation, that is the tendency of all prices and wage rates to rise. [1]
Everybody knows that the Austrians believe an increase in the quantity of money inevitably leads to an increase in the price level. It's got nothing to do with the fact that, anywhere in the world, and regardless of what you may think causes the price level to rise, an inflation rate of 2% is understood to mean that the price level has risen by 2%, not that the money supply has risen by 2%. As an example, the money supply (M2) grew by 25% in the US in 2020, yet nobody claims that the inflation rate was 25%. And so this is what the term inflation means, like it or not: an increase in the general price level. This is pretty basic stuff. Even your own quote acknowledges that this is the modern meaning of the term.
When Austrians talk about inflation, they specifically mean an increase in the money supply. They also mean this today. You said in your original post that for a cryptocurrency to be inflationary/deflationary, it needs a governing body adjusting the supply - which is not correct using the original definition of inflation. And even if you use your definition (increasing price level), it is still just an effect of inflation (increase in money supply) that may or may not be caused by a governing body.
To be clear, that inflation means an increase in the price level is not my definition. It's the generally accepted definition. The fact that a fringe group known for their crackpot ideas and their rejection of empiricism, representing <0.00000001% of the world's population, may or may not (even that is debatable) use the term inflation differently is irrelevant. So what are you saying? That it is possible for the price level to increase or decrease at a fixed rate without a governing body managing the money supply? How would that work exactly?
[1] https://www.core-econ.org/the-economy/index.html
[2] https://www.core-econ.org/the-economy/book/text/13.html#138-...