I think risk is the difference. Someone invested (esp. heavily in one name) who borrows against that holding to finance consumption is basically adding leverage to their position, increasing their risk in order to continue growing their investment. Since growth is a social good, that's incentivized. It becomes income when you take your chips off the table i.e. making that capital entirely private instead of investing alongside others.
The risk is less apparently in TFA since it, you know, focuses on 25 of the absolute wealthiest people in the world, any one of whom can finance any sort of consumption short of becoming a nation-state without impacting their position, borrowing or selling.
Whether _wealth_ inequality is so undesirable that it should be ameliorated even at the expense of growth is a policy conversation worth having, but equating change in wealth to income and arguing _income_ inequality doesn't really hold water, esp. for wealth primarily invested in public names