If there was no difference between capital and labor, then capital gains and labor income would be taxed at the same rate. That's just the empirical argument. The theoretical is left as an exercise to the reader.

I feel like you have only a cursory understanding of finance, economics, and taxation. If you didn't, you would't ask questions such as

if someone owns a food stall, are they capital or a worker?

It reads like you're trying to find evidence that reinforces your priors while dismissing whole swaths of empirical and theoretical work that would immediately challenge it.

For context, I spent a decade as an M&A banker, so as far from a Marxist as one can be.