I think he understands that but when you have a situation where the real economy is growing only around 1% per year but rich people grow their wealth somewhere around 4-10% that can only come at someone else's expense.
In other words while the real economy might be growing as a "non-zero sum game" the growth of the elite and the rich far exceeds that and outpaces it. Net effect is that their wealth is wealth away from everyone else.
Stock market/trading is a kind of zero sum game. For him to gain, someone else has to lose.
However, real economy is not a zero sum game and I dont think he understands that. AFAIK never was an entrepreneur, or created a business.
He advocates for a wealth tax, ie a tax on unrealised gains.
For realised gains, we already do wealth tax and thats called capital gains tax.