A dictionary definition of "Income" is "money received, especially on a regular basis, for work or through investments". It's not just the narrowest definition, e.g. "only the thing that appears in the Income box on your [US] tax form, under the latest revision of the US tax code". (Like when Mitt Romney in 2012 accidentally shone a spotlight on "carried interest" which allows private equity managers to pay only 15% tax.)
If your unrealized CG is so high that you can live off the loan until you die or retire, then it semantically isn't really a loan, is it.
When US CEO compensation becomes so excessive that it means that something that originally meant temporarily deferring tax in the 1920s now means deferring essentially all income taxes for most of their working life, it does suggest things have gotten distorted.
Sure ProPublica should have been more accurate and said "Here's stuff that is functionally equivalent to income, yet isn't taxed". But if you want to argue that ProPublica isn't entitled to is own semantics, then neither are other people in that debate. For example, we'll probaly see more scrutiny that Clarence Thomas (and his mother) were clearly receiving multiple streams of undeclared "income" or benefits from Harlan Crow. Also allegedly a complete miscarriage of ethics. (Yes that's currently primarily an ethics scandal not a tax evasion one, although it might become that too).
By the way, Warren Buffett essentially made the same point as ProPublica with his famous 2011(?) wager about CEO's secretaries' tax rates being higher. Noone accused him of a complete miscarriage of ethics.