Fascism is definitionally capitalist

That really depends on how you define fascist and capitalist I guess.

Here’s a definition of facism from Webster:

severe economic and social regimentation

And Cambridge dictionary:

state control of social and economic life

And the definition of capitalism in those same dictionaries. Webster:

an economic system characterized by private or corporate ownership of capital goods, by investments that are determined by private decision, and by prices, production, and the distribution of goods that are determined mainly by competition in a free market

And Cambridge:

an economic and political system in which property, business, and industry are controlled by private owners rather than by the state, with the purpose of making a profit

Both definitions of facism say it involves strict, top-down control of the economy, and both definitions of capitalism say it involves private (non-government) control. When you get too much government control, it stops being capitalism and starts being mercantilism:

Fascism and totalitarianism adopted mercantilism in the 1930s and 1940s. After the stock market crash of 1929, many nations turned toward trade protectionism to save industry and jobs. They responded to the Great Depression with tariffs and trade restrictions.

In fact:

Both Russia and China continue to promote a form of mercantilism.

The article goes on to say:

In response, leaders like U.S. President Donald Trump advocated expansionary fiscal policies, such as tax cuts, to help businesses. He also argued for bilateral trade agreements between two countries, rather than multilateral agreements between many countries. Mercantilism opposes immigration because it takes jobs away from domestic workers. Trump’s immigration policies followed this mindset.

And just not just Trump, look at Biden’s tariffs against China, those have the goal of increasing domestic production of EVs in a protectionist move, which is very much mercantilist.

When people say “capitalism is the problem” or “capitalism leads to fascism,” what they are usually talking about is mercantilism. That’s the marriage of government and capital, where government interests are served by steering the economy in a certain way, and capital benefits from the strong arm of the government protecting their profits.

So the problem here isn’t capitalism, it’s government control over the economy. Governments should largely restrict themselves to welfare, consumer protection, etc. The more regulations and whatnot start to look like mercantilism, the easier it will be for a strong individual to steer the country toward facism.